here is my question though. Why not just divide it?
Exactly, why not? You can use clear dividers(with the clear dividers, its less noticeable, creating more of an illusion that they aren't separated)and separate the tank into four sections, and each section will be a little under 20 gallons still I believe....enough to have a small community with each betta. You could have corys in one section, neons in another....ect. You get the beauty and space efficiency of having all your males in one tank, but there isn't the risk of them tearing one another to shreds AND its like having four little tanks in one! ;)
I gotta play devils advocate but I'll repeat that you shouldn't do it.
But people tend to think more than one is a instant dead/ unhappy betta. With a large tank with lots of plants, caves, rocks, barriers, wood, etc. it is perfectly possible. That being said it shouldn't be done just because you've had bettas for a while. I think its a cool idea but I wouldn't try in even.
In the wild there are multiple males, and they see each other. Males don't fight to the death to fight to the death, they fight for territory. If a tank is large enough that they have seperate territories and all lines of sight between the males is possible and should result in fighting because the male intruding will most likely retreat not fight.
Now that I'm done with that, do not put 4 in a 75. A 75 would be the absolute minimum for 2 IMO and you'd need more experience than just keeping betta. You need to know how to set up a biotope properly so they have territories and broken lines of sight. You need to know what else can go in there, etc.
It's possible for someone to fly a huge jet over the ocean, but that doesn't mean you can because you've flown a small plane.
I'm not advocating it, but I think it is important to give true advice. It is possible, not I wouldn't recommend it.
Your 100% right chicklet, they are contained but at a point they can get away. A male will defend his territory, they aren't a fish that will chase down and kill a fish to eat it, they will defend their territory but once they feel it is safe they will stop, and usually it won't go any farther than flairing.
Like I said it's not something 99.9% of betta keepers should ever do, but you shouldn't right it off as crazy.
If you are really set on it I would strongly recommend one of the type of wild betta that can live together. If you aren't willing to spend the money to get some of them then you probably aren't willing to put the type of money required to make a 75 acceptible for undivided males.
I understand the appeal of a natural looking tank that has muliple males, just like I understand the appeal of trying to build a home made jet pack. But just because its appealing doesn't mean its worth trying.
I agree with pretty much everyone. I don't know why you'd even want to risk it knowing that it could end in death or a bad wounding. Put fish in your tank that won't run the risk of killing eachother. The fish have no choice in the matter so we're the ones that have to make responsible choices.
I'm normaly keeping two males and eight females together in a 200litre tank (think it is about 50 gallons) and that have never been a problem. But I wouldn't let any betta into the group without "knowing" him/her a little bit, that's to risky. For example there is gonna be a fight about status if you choose two dominant males, and that is certainly not anything you want. And If you choose to keep several males together, always have a backup plan, and be careful to watch them constantly atleast an hour when you let them together, and go check on them often, minimum once every half hour the first days. You can't be sure that they won't fight until you see them swimming near each other whitout flaring even a little bit. In the beginning there may be a little fight(you know, flaring and looking mean), but If they start attacking each other then you have to separate them directly.
I'm a little sceptical to this idea, but you seems to have your mind set up. At least you're not jaring them up for a life time. Please just be really careful, and choose your bettas wisely. Pretty obviously it's a bad idea to chose bettas who flares at their own reflection. And keep the tank full of plants, atleast 50% of the tank should be full of different plants and there should be lots of hiding places built of rocks and roots. One great tip is to have pistia stratiotes, I would never have bettas together if it wasn't for the good hiding space the roots gives.