A preface: if you're well into Granblue Fantasy, this probably is going to be redundant. I'm mostly posting this as a guide post to people who are just starting to get their third row classes and are wondering about the next step there. Possibly for those who are aware of Row IV stuff, but haven't really looked TOO deeply into it as well.
BASIC RUNDOWN
That said, let's quickly run down the basics on the little intricacies of classes. I'm going to assume you know about the difference between classes and their basic abilities. Stuff like Dark Fencer being best for solo battles, and the like. But maybe you're not really solid on things like Mastery and EMP.
Class Mastery is a bonus applied to your MC upon hitting level 20 of a class (it does not apply to your other party members), and when you gain experience with a mastered class you'll earn Extended Mastery Points (EMP), otherwise known as Zenith Points (ZP) with the Japanese side. EMP is used for upgrades on mastered classes, both in stats and in learning new abilities. Your EMP pool is shared between classes, but once spent in boosting stats, those bonuses will apply only to the class you spent it on. You have a limited amount of boosts you can give a class (represented by your Extended Mastery Level shown on your class menu), but this limit will grow as your rank goes up. In general, you can get two more stars per 10 rank levels.
As a digression on Extended Mastery Abilities, these abilities will only work on classes along their line. If you get Quick Down in the Dark Fencer line, that can be equipped if for some reason you wanted to use it on Enhancer, but you can't use it on Weapon Master. Not all of these abilities are useful, but they generally have at least a few applications. Of note, Bishop's Veil is very useful if you don't have a character with it, Hawkeye's Break Keep is great in Impossible (HL) difficulty battles, and Sidewinder's Change also has applications in farming renown.
But let's move on to Row IV classes, and all that they entail. Before anything, let's talk about how you get them. You've probably looked at them and blanched at the price of 2500 CP. Sadly, this is the least of your worries. Beyond those odd 'Distinction' things, which I'll get into later, the other requirement is an odd little trophy that's along the lines of 'Superior Something'. To get this trophy requires an incredible journey of grinding that's outright impossible to complete until you're rank 101 and above. While I'll get into the details, what this basically means is that you are extremely unlikely to even be close to touching these things until you've unlocked and mastered pretty much all the other available classes, no doubt including the Extra classes (more on these later).
That said, the fact that you need to be rank 101 or above does not mean you can't start prepping for it. If you've gotten to the point where you've unlocked and mastered all Row III classes, and are at least rank 60, you can try your hand at this preparation. This brings us to the major topic of this thing, Class Champion Weapons.
CLASS CHAMPION WEAPONS
Class Champion Weapons are largely the real obstacle in getting that annoying Superior Something trophy, and unlocking Row IV classes. To make them, they require some materials you literally cannot currently obtain until you're rank 101+, hence the impossibility of unlocking these classes until you're at that rank. To be honest, even when you're at that rank, there are a few battles for these materials that you're going to have a hard time with unless you've been perfecting your weapon grid like crazy. We'll talk about that later, though. First, let's talk about what these weapons are.
As the name implies, Class Champion Weapons cater specifically to certain classes. There are, in fact, one class weapon to every class (in this case, class lines such as Enhancer-Arcane Duelist-Dark Fencer-Chaos Ruler are counted as a single class). Extra classes are included in this total, so there are seventeen class weapons altogether. They all have different unique abilities attached to them that cater to their specific class. For example, the Priest class weapon Nirvana boosts the effect of Panacea and in its later upgraded ability gives the MC an Auto-Revive on using Panacea. These abilities only work with the weapon in the main hand, however.
Not all of these weapons are equally useful currently, and given the grind required to fully upgrade one, they are not something you choose lightly. I'm not going to get into this subject much since there is a layer of subjectivity but in general, the ones considered most useful are Nirvana (Priest), Oliver (Bandit), and Nebuchad (Gunslinger). Nirvana because it's simply very powerful ability-wise, Oliver is useful because it gives boosts to Treasure Hunter, and Nebuchad is one of the best guns for the Gunslinger class outside of the Revenant (Guild Wars) Weapon gun.
Moving on, let's talk about how Class Champion Weapons are obtained. They are specific 'Fabled Weapons' in similar lines to Bahamut and Revenant (Guild Wars) weaponry. Unlike the other two, however, you need to start grinding for it in co-op missions. While you can, and in fact will need to gather many materials from outside of co-op to get and upgrade them, the necessary materials for the first level of the weapon are in the later stages of co-op, in the Extra stages to be specific. What exactly you require will vary depending on which weapon you decide, but as a general point, the co-op drops you require are 35 Creeds, 30 Normal/Hard Gold Chest items, 70-90 Showdown-type boss items, and 30 Grimoires.
To go into a bit more detail on these drops, Mage and Warrior Creeds will drop in any Extra level difficulty battle at a fairly low frequency. They will also drop in Normal/Hard difficulty, but at an even further, almost legendary low frequency. The Normal/Hard Gold Chest items I speak of are the rare drops that sell for 5k rupies each, Vortical Pinwheel, Searing Stone, Resplendent Coral, and Opulent Amber. The Showdown-type boss items are drops from the Extra stages 2-2, 2-3, 2-4, 3-2, 3-3, and 3-4. They're alternates of the rotational Showdown bosses such as Ifrit and Corow, and drop co-op unique versions of the same items you get from their regular rotation battle. The Grimoires are items that drop from the Extra stages 4-2, 4-3, 4-4, and 4-5. Beating these bosses is a high task, but in general there are many high level players that would gladly carry simply because it meant more experience and treasure for them. So the only real requirement for farming these items is being rank 60 (because the later Extra stages are restricted to this rank), and a lot of AP to host these battles
But let's step back a moment. How are the Class Champion Weapons obtained at all? You first need a replica. These can drop in Extra co-op stages, but you can also simply purchase one from the shop. Under the Fabled Weapons option you can click on the Class Champion Weapons, and then Redeem Replica Items option. You need to exchange several Champion and Supreme Merits for a Legendary Merit for it, as well as one Creed and Class Distinction, but you can get up to three copies this way. You need to uncap the replica fully for it to be upgraded, however, so you need either 3 more copies after this, or to exchange for three Steel Bricks from the casino.
Once you've obtained the replica, uncapped and leveled it to 75, you can then upgrade it to its second level. This requires the element boss items, the Gold Chest items, 10 Creeds, 40 Antique Clothes, 200 Prism Chips, and 5 Blue Sky Crystals. Once this upgrade occurs the replica will become an SSR level weapon, and gain its second ability (which of course varies depending on what weapon it is).
After its upgrade, now you start to go into rank 101+ territory. At this point you need Class Distinctions, a lot of them. 20 Distinctions are required for unlocking any of the classes once you've gotten that Superior Something trophy, but you also need 40 of them to complete the Class Champion Weapon. There are about three ways of currently getting a Distinction. The first is getting the co-op trophy by playing a certain number of missions as a certain class (e.g., 100 wins as a Hawkeye gets you 1 Bandit Distinction). You can only get a total of 3 Distinctions this way, and you probably already used one of these for getting the replica. The second is as a drop in co-op. There is a very rare chance, even lower than creeds in normal/hard difficulty, of a distinction dropping in a co-op mission. There are 17 distinction types, however, and no way of you farming for that type in this manner. The third way is through prestige pendant exchange.
You've almost certainly been using the renown pendant exchange at this point, so you should be vaguely aware of the option for them in the same area. The system for them is pretty much exactly the same, only prestige drops from Impossible difficulty level battles, which will only be available to you at rank 101+, hence the obstacle. You have a weekly limit of 500 prestige, and it requires 20 prestige per Distinction, with a limit of 1 Distinction per type every day. So all you have to do for this item is just fight, and exchange for them. Once you're actually rank 101+, this is easy, you just need to be sure to remember to trade for the one(s) you want every day.
So, let's say you're at rank 101+ now, you've got all the Distinctions you need, you're ready to upgrade your Class Champion Weapon. What you need now is only 10 of those Distinctions, 256 Weapon Stones, 50 Elemental Quartz, 25 Creeds, 30 Champion merits, 120 Prism Chips, 2 Steel Bricks, and 5 Blue Sky Crystals. You should already be aware of most of these things, but for the more out of the ordinary things... You generally are going to get Weapon Stones and Elemental Quartz from reducing SSSR weapons and summons. Premium draw items of such items will give you higher amounts of the stuff per reduction. The exact stones and quartz you need will vary depending on your weapon. As an example, Oliver (the Bandit class gun) needs Gun Stones, and Wind Quartz.
With these items in hand, you need to have also leveled your Class Champion Weapon up to 100 from its former SR limitations, and then you can finally upgrade it to its second form. But there's one more step, you need to element change it to complete the process. Even if you like its current element, you still need to element change it to complete this journey and allow you to get those Superior Something trophies.
At this point, you have even more rank 101+ grinding to go. That said, if you've gotten to this point, you've likely already got all of the required materials anyway. Depending on the weapon and what element you're changing it to, the items you need vary, but in general... You need 30 class Distinctions, an additional 512 Weapon Stones, 30 Element Centrums, 30 Grimoires, 200 Showdown Silver Anima, 3 Silver Centrum, and 15 Blue Sky Crystals.
A lot of these things you should already be aware of, but I'll clarify on one point before expanding on the rank 101+ stuff. The Showdown Anima in this equation are the ones that drop from the actual Showdown in rotation, not anything co-op related. You need 200 of them, and which ones you need depend on what element you're changing things to.
Now, onto the items you might not be aware of. Centrums are more rank 101+ items. The Elemental Centrum are a guaranteed drop from the Impossible difficulty versions of the Omega family (i.e., Tiamat, Leviathan, and such). You will no doubt have already gotten more than enough of these while grinding for the prestige for your Distinctions, unless you decided to ignore the element you wanted, or spent all of them on upgrades for different things. Silver Centrum is another matter, however. These things are much harder to come by. Currently, there are only three ways to get one: they drop in Impossible difficulty Rose Queen battles, they are available in share chests from Grand Order raid battles, and you can exchange 5 Heavenly Horns for one of them per month from the shop. Heavenly Horns are a rare drop from Grand Order raid battles, but you can also exchange for one per 5 Azure Feathers (which you will get as a guaranteed drop from Grand Order raid battles).
Now, I'm going to digress a bit, because I want to actually put this in perspective to people. Impossible/HL difficulty battles are generally kind of a distant thought to anybody grinding out Omegas, so a lot of people probably haven't looked into what exactly they entail. The major difficulty with them is not a really an increase of difficulty (although there is that too), it's that raids get far, far more limited when you go into Impossible difficulty. Impossible level Omega battles are limited to 18 participants instead of 30, and then you go a level up into the Summon category (such as Nezha, Olivia and the like) and are limited to only 6 participants per raid. Battles are less about being able to actually deal damage (because that should sort of be a matter of fact at this rank) and more about actually coordinating with your allies, because you are going to need certain abilities in battles at certain times, and if someone screws up and dies, you might have very well lost something integral to beating the raid. You also cannot use elixirs, so there's no way to make up for mistakes. Obviously some of these battles are easier than other, allowing for more leeway, but the point is that the difficulty is not about power, but of strategy, and that's not necessarily an easy thing to coordinate for many people even if you speak the same language.
The major point here is that, likely, your best option for getting Silver Centrum is through Grand Order raids, which open up at the same time Bahamut raids do. If you've got your eye on a Class Champion Weapon, you also want try and do some of these battles. If you're unlucky, you're going to need to do 75 of these battles to get enough to exchange for 3 Silver Centrum.
But enough about that. Let's say we're here. You've got all the materials you need, you element change your Class Champion Weapon, the end of the journey has come. Well done. It's time to kill your master.
ROW IV CLASSES
Change to whatever class you're looking to open up (e.g., Dark Fencer for Chaos Ruler), and journey to your MC's home, Zinkenstill. You're going to find a new quest that'll open up, and you'll need to beat an opponent with only your MC in your party. Whatever element you are, the opponent will be your weakness. Give it your best, and you should be able to win without a problem if you've come this far. Upon winning, you will get the Superior Something trophy related to whatever class you were. You can repeat this quest with all the different classes to get all of the trophies. Currently, you do not need to make more than one Class Champion Weapon to unlock the Row IV classes.
And then you're done. You still need 2500 CP, and 20 Distinctions for every Row IV class you unlock, but at this point that's unlikely to be a worry either. The major advantage to these classes, besides the new abilities, better stats, and further mastery bonuses, is the ability to equip multiple subskills. You can still only equip up to 4 abilities, of course, but Row IV classes only have one set ability, so you just equip whatever you desire in the other slots. Versatility is an amazing thing. Each of these classes also have useful Extended Mastery Abilities, almost all of which are incredibly strong.
EXTRAS
But let me go back, back, back to something I mentioned previously and glossed over since it wasn't really overall relevant, and there are better explanations around for it. For completion's sake, I figured I should talk about Extra classes. These things are co-op centered. If you've actually gotten to the point where you're thinking about Row IV classes, but haven't looked into it all THAT much, Extra classes are actually the next step for you. Unfortunately, they're generally not very good.
Each Extra class has something of a niche to it, but the majority of them have largely useless niches. I'm not going into huge amounts of details, again because there are better guides on these things, but while there is no absolutely useless class, very few of these classes do things that something else will not serve for, and better at that. Largely the two Extra classes of any real note are Ninja and Gunslinger. Ninja has its niche in being able to inflict paralyze on enemies which is the best status in the game (albeit can be misused into tragedy). Gunslinger is a class that can be incredibly powerful, but requires almost as much grinding for it as the Class Champion Weapon. I largely can't speak on the subject of it because it never interested me, but if it does you, then check around, there is a wealth of knowledge on the class.
Largely, what you want out of Extra classes are the mastery bonuses. Many of them give incredibly useful things such as increased skill damage, attack, or even an increase to building up your charge attack bar. This is not to say that there is nothing good out these classes, even excluding Ninja and Gunslinger, there are points you might want to use these classes. Mystic's ability allows you to clear weak mobs faster in farming scenarios. Alchemist's Extended Mastery Ability allows you to boost attack up across to other raid parties. I personally found the Sword Master's abilities very interesting and powerful, although not necessarily more useful than the general things in Row III jobs' hands.
The only caveat to Extra classes is that they require you to clear Extra co-op stages, and creeds to unlock. This means they're a small competition for your Class Champion Weapon, because you overall need 28 creeds (11 mage and 17 warrior) to unlock all of them. Since Creeds drop rates are rather low, and you need 35 of one type of Creed for your Class Champion Weapon, this can increase the time spent farming for things in co-op. Just something to keep in mind.
OTHER FARMING
As mentioned by Sun-Tail in the comments, there are some shortcuts to getting a few of the other items mentioned in the above. In the casino, you can exchange chips for four copies of an SSR gun, and four copies of an SSR sword. The gun will initially cost 77,777,777 chips due to it being an exchange for the SSR character Christina, but subsequent purchases will reduce the cost to 7,777,777 chips instead. The SSR sword only costs 2,500,000 chips per copy. The reason to get these, as their abilities are very lackluster, is that they count as premium items in for reduction purposes, and thus will give a rather large bounty in weapon stones. If you're going for the sword or gun Class Champion Weapon, these will ease the grind of getting so many weapon stones.
The other more mundane items such as Prism Chips and Champion Merits will come in time when you're grinding for the other items. Grand Order will most certainly give a number of Champion Merits, for example. While it isn't related to the grind for other items, doing Proto-Bahamut battles is also another good way of gaining Champion Merits.
Prism Chips can be exchanged for in the shop via its larger cousin Flawed Prism (up to 30 every day). The most convenient way to get Tomes outside of grinding is exchanging Cerulean Stones for them, as you can get 10 Tomes for every Cerulean Stone.
Antique Clothes drop from the quests in Former Capital Mephorash. The likely the most surefire way to get them is from doing the 80 AP-cost quest "Antiquarian Troubles". You can only do the quest twice every day, but you are guaranteed 5-7 Antique Clothes each run.
RESOURCES
For more information on the class weapon materials, please refer to this google spreadsheet. Specifically check the the 'Class Weapon Guide' and 'Class Mat. Checklist' tabs. Alternatively, the Granblue Wiki has similar, useful information on the subject.
For more information on Row IV and Extra Classes stats, please refer to this guide.
For more information on Gunslingers, please refer to this guide.
TL;DR
Do co-op, do grand order, do showdowns, do impossible battles, get class weapon, upgrade class weapon, get new classes.

Las Vegas as a destination...sucks. It seems like like the hotels nickel and dime people FOR EVERYTHING. Want to use the pool, or the gym, or the concierge? Well, today is your lucky day! The hotels took all of the services and activities that used to be FREE and used them as an excuse to charge a resort fee... If that wans't bad enough, now the hotels are charging for PARKING! But don't worry, they are only charging so they can provide you with new infrastructure and better service... if you believe that I know a Nigerian prince that would like to talk to you.
So you payed to park, you paid for your over priced room, and finally you paid the stupid resort fee. Now it should get better right? I mean, you payed so much the hotels have to offer something right? NOPE! Gone are free buffet vouchers, gone are free drinks, gone is pretty much any sign of value. The drinks at the casino bars are about 4-5x overpriced - NO, I don't want to pay 22 dollars for a real cadallac margarita (patron gold, lime juice, Contreau w/ Grand Marnier. I was told that Vegas is transitioning from a gambling destination to luxery nightclubs and high end pool parties. They wonder why people don't gamble as much, sure the economy plays a part but the main reason is: no one wins anymore!! The slots and other machines have been tightened down to the point where not only do you expect to lose money, it isn't even fun anymore. Put $100 bucks in lose it all before even 1 "bonus game" comes up. Don't get me wrong, you will "win". Ya, you "win" $0.15... too bad the spin cost ya $5. The casinos don't bother to run any worth while promos anymore, the slot and video poker machines have been tightened up, and the casinos don't comp shit anymore. Instead of a free buffet, now you get 10% off a buffet! Instead of a free night, you get a "Super Special Player's Club Rate" (great, 8% off...on the weekdays).
So back to this Luxery night club scene. Here is a Vegas night club explained: Charge guests at least $30 to get in, drinks have to be $20 each and be light on anything that is not RC Cola or cheap sugary syrup, for music - choose between electronic that all sounds the same after 45 minutes or whatever rap songs that everyone heard 1000 times on the radio making the drive, Man... really? Am I the ONLY one here that is sick of nightclubs?
I can't be the only one that is sick of these "cool" hotels. The only guy I know that brags about staying in the [Fancy Pants Hotel] is a 350 fat guy who posts quotes like "It's hard being friends with girls when all they want is sex" on Facebook daily - who although he swears he has "boned a ton of super hot girls, some models even", he has NEVER had a GF. In fact, no one has EVER seen him spit game, touch a girl (the notorious arm gram doesn't count, might even prove my point further), or bring a girl home (Maybe you have met his brother - the guy that is always showing of his bottle of Cristal, the same one he has had for 5 years...) . However, it seems that there are enough of these losers dying to impress women by their baller economy room with view "penthouse" that go to those Luxury nightclubs to make Vegas turn into douche bag central.
Sure, there are still a couple of good values left - but they are dying fast. One can get 4 real Patron Cadillac margaritas for the price of one at [fancy pants hotel] by taking a trip down the strip to Circus Circus or Stratusphere. If you can stomach the smell of regret and cheap hookers. The Hard Rock is still doing the 777 deal, however the cab ride is probably going to cost ya triple that each way. The Casino Royale has Michelobe Ultra's (a type of beer that no one knows exists outside that small casino) and what they call "margaritas" for $1. Plus they have the only 100x odds craps on the strip (which is actually pretty damn good, craps has one of the lowest house edges available). The Magnoila cafe has a descent prime rib dinner that with coupon comes to ~$10. Sure one can go downtown and have some fun for less, but lets be honest. If the fancy lights aren't above you, you have a good chance to getting robbed/mugged/assaulted.
The sad thing is I don't see the last good values lasting too long. Iconic hotels like the Riviera and the Sands is just the beginning, in a couple more years all the values on the strip will be replaced with "luxury hotels" following MGM's lead: Charge for Parking, Charge Resort fees, Charge for what used be complimentary. RAPE people for nightclub entrance, nightclub drinks and Shows (although last time I went I did get a good deal on Blue Man Group and Phantom of the Opera tickets...but only after I spent hours looking). One has to wonder, what the fuck are they going to charge for next? House keeping? Towels? Soap and Shampoo? Is there going to be a curtain over the window next time I go, with a dollar fee to open for 15 minutes? Is the AC going to be p4p?
Sorry for the rant, I just honestly think that resort fees and having to pay for parking in a place like Las Vegas should be criminal. I remember my mom grabbing a handful of my quarters (that I was using at the ARCADE) to play the slots while waiting for my pops to come down from the room and winning 400 in quarters. I remember my neighbor that would get so many free nights he couldn't possibly using them all just from comps from buying in $800 at the poker table. I remember my friends parents going to Vegas every weekend they could, and winning. They used to keep track of their winnings vs. money spent on gambling, room, food, etc. They said they hardly ever had years where they were in the red more than 1k - and most of the time they either didn't spend anything or came home with more than they left with. Those same people not only don't keep track anymore - they have basically stopped gambling. They both played Video Poker, and it has gotten to the point where they don't even enjoy it anymore. Last time I saw them in Vegas we met up at a hotel for dinner. I went to get my friend, her dad and I drinks from the bar (because all they would comp was "mixed drinks", that could best be described as a splash of the lowest of the low plastic bottle rum or vodka - honestly can't tell which its so god awful - combined with "soda" that makes those shitty bargain brand sodas seem like nectar of the gods). I was gone no longer than 5 minutes, and when I returned her dad wanted to leave. He had lost almost $300 dollars in video poker...in less than 5 minutes. He wasn't playing a crazy high roller machine either, he was playing $0.50/credit and 5 credits. He didn't win (not counting pairs) one hand. He didn't even get dealt a 3 of a kind (which anyone who plays video poker knows, is one of the most exciting things because your praying for that 4th one and a nice win!) once!
Tl;Dr: Vegas sucks. With the addition of resort fees and parking fees and the transition from a gambling destination to a luxury night club destination it is quickly turning into tool central.