I highly doubt you can fit four 15's back there. Like mentioned, you'll probably be better off with a pair instead of trying to cram 2lbs of crap in a 1lb bag. Still, we need dimensions.
That's the problem I can't get them I'm trying to see what all I can do to before I get it, I know it's possible because they have an impala with 4-15s but it's possible because the Cadillac is bigger. It's like 22 cubic feet. But what are better subs at the price?
Also, you won't get 22 cubes in that trunk. And just cause someone crammed 4-15's in a impala trunk doesn't mean they are in the right size box either.