1- 15'' or 4- 12''

4 12's have a total surface area around 2.5 times more than one 15, so in theory the 12's would be louder with the same power.  It sort of depends on the particular subs because excursion is also a factor as well, but i would guess in almost all cases the 4 12's would be louder.  I used to run 4 12's on a 1000w and it sounded pretty good, but I went to 2 15's on 2500w after that so I can't really make a comparison there.

I thinnk I can answer this question a bit more. See my first system consist of four pyrmid original 12's  pushing about  800 RMS to all four subs. Then I got a single Kicker L5 15' pushing the same 800 RMS and it performed way better than those 12's ever could.  So in theroy  four 12 should be louder than a single 15, but just like what fakepete said, there are other factors that go into this besides power handling. So I guess what type of sub's you use are important too.live & die 4 tha luv of BASS!!!!