The main idea behind the Swarm or distributed bass array is that while any one sub is going to produce lumpy modal bass response, a whole bunch of them in different locations will produce a whole bunch of lumpy modes that combined together result in smooth, deep, articulate bass.
It works. Tim and Duke and often times even me will go into all the many solid reasons why- the physics, scientific research, human hearing physiology, etc- which is certainly nice to know. But honestly, it works so well you will be 90% of the way there and pleased as punch even if all you do is plop em down wherever they go, twist a few levels by ear, and sit back and enjoy. Its that good. Because you have science on your side.
Briefly, its not that EQ won't help. A little EQ can always tweak things a little better. But the main reason people think they need EQ so much is because they are using just one or two subs and getting the awful response one or two subs always give. Soon as you go to four (or more!) a lot of that need goes away. What little you are left with can be easily handled with the level adjustments on each of your subs.
Again, yes you will be able to get it "more perfect". But the improvement from four subs vs three is so much that if it was me I would take whatever money I was thinking of spending on EQ and just buy another sub. Or two.
Another thing is, its really hard to ever have too much bass. Don't misunderstand me. Not saying you can't turn the bass up too high. You certainly can do that. What I mean is it is really hard to have too much low frequency headroom. Vast majority of the time, vast majority of recordings, there's hardly any really low bass. It is as if your awesome Swarm subs do not even exist. Every now and then though, you play the right thing loud enough, its freaking insane how much bass energy there can be! It was always there, you just never knew it, because even with one or two really good subs they just never got it to a level you could notice. Seriously. Get four. You will see.
So in other words if you can buy four awesome 15" subs then go for it. But just keep in mind, the improvement from four is so great you will probably be better off with four less awesome subs than two or even three really awesome subs.
One real life example since this is a hard concept to grasp. There was a guy here with four ordinary two-way speakers. Not subs just ordinary 8" or maybe 10" two-way speakers. He asked what's a good way to hook em all up. I said two normal stereo, use the other two like subs, down on the floor facing the wall. He was blown away how much smoother, better and more articulate the bass got. So the Swarm concept is so powerful you don't hardly even need good, just more.