I went from no subs to a Distributed Bass Array and I will never, ever go back.
My system was set up professionally. Meaning, I paid someone who knows what they're doing and he used acoustical software to dial everything in. There was no guessing where to place the subs or how to dial each one in.
Of course, you can do it yourself. I'm sure many have and there are instructions for doing so. So I'm not advocating you pay someone to do it. I just know my limitations and what I wanted.
As for the reasons four is better than two (and two better than one) there are different, technical resources you can look at. Check out jamesromeyn.com and you'll find the theory behind it.
You're half-way there. Like I said, two is definitely better (IMO) than none, so you may be happy right there. But, if you go further so will your entire system and musical experience.
In short, I am 100% behind the benefit and value of a DBA. That's a big "yes" it's worth the expense and effort of the "extra subs".