What you need is a way to measure your system if you want to optimize it. That means you need a mic. You can kind of do that if you have a good smartphone and the right app, but I wouldn't trust that as much as a decent mic. Anyway, if you like the sound and it sounds good to you, you don't need to fret about it too much.
If you are going to add more subs later, and you have freedom of placement for these subs, ie. there are many different places you can put them, its worth getting a mic and testing different locations to get them in the best spots. With 4 subs and the right placement, you might be able to get the frequency response so nice that there would be no need for any EQ whatsoever. EQ might actually hurt more than it can help by amassing group delay.