We can ship a box for 4 12's, thats really no problem.
As far as 2 15's being louder then your RE SX's - there are so many things that can factor into a setup being "Louder" than another - we never will tell you one thing is going to be "Louder" then another. We can tell you that the custom boxes we do build and ship with our subwoofers are extremely high quality and built specifically for the SKAR subs for maximum performance.
We do invite you to try 4 15's out, just shoot us a PM or email!