well after just doing this i can tell you

1st at least 2000 rms a sub

2nd 3-4 batterys per amp

3rd 200+ in a alt

4th a battery charger 200amp or better alt wont keep up

5th 20-25 sq foot of space for cabnet at 32 hz

6th a glass company on speed dial

4 15's in your car can be done, but BTL's IMHO were a bad choice. You're going to need 1.5-3KW per BTL, probably a dual alternator setup, and MULTIPLE batteries. In your vehicle SSD's would have been more economical based on the amount of electrical support you will require. If it were me, I'd run 2 of those BTL's with about 1.5K each (appropriate electrical support of course) and then the other two in my daily driver. I don't know if you've ever heard a car with 4 BTL's properly powered, but I would bet money that it would be too loud for you to sit inside....comfortably .

In my hood, lot of people would brag about their 4 15's or 6 12's cuz it sounded good to say that "I got 4 15's". But mostly they are P.O.S. subs with sony Xplod amps and just an under hood battery. With 1 BTL18, a decent box, a true 2-3KW and the right electrical you can probably out knock most in your town....just my .02