Why are you not considering mounting the amp(s) under the front seats? That's the place most forum members seem to use.
4" coaxes in the rear pillars will work fine as long as you something like what I did
here. There's no rule that says 4" speakers are bad, but 5 1/4" speakers are good. 4" speakers have a bad rep because they're usually mounted in compromised locations with no baffle. Add a decent baffle, and they can sound worthy of their brand name and the $$ you spent for them.
