its NOT necessarily better -sometimes ppl get a deal on 4 & they buy it . Some ppl do it for looks - aka "more the merrier" Some brands the 12s are actually cheaper than the 15s .Another reason is that their amp can push 4 samaller 12s & 2 bigger 12s/15s .
2 really good quality subs are plenty enough for most normal ppl .....
The subs are going to react differently and you're going to have phase differences. It's generally not a good plan to run different sizes, brands, or models of subwoofers; you want all of your subs to act as 1 consistant air moving unit.
you could probably get by w/ mixing subs w/ a bigger difference in size EX: 10's and 15's or 8's and 12's.it would work well if you set the crossover on your amps to blend them. set the crossover for the 10's and cut it off at the low FQ's and the 15's could be set to run the ultra lows. I used to have 2 10's and a 15 and it actually worked well.