Hi Bully, sorry for asking lots of questions but cant help unless i know what you have and what your looking for.
The w6's can work in a 'relatively' small enclosure so you might be able to squeeze them all in, to be honest i probably wouldnt bother unless you are losing the rear seats or felt like going down the wall route? and can afford another amp.
Thought about selling the other 2 subs to fund some of the install?
What model number amp have you got?
You'll need about 5.5cuft to run two subs ported half that to run them sealed which may allow you to fit 4 in.
Personnally i;d rather run 2 subs in bigger ported boxes than squeeze 4 in with less power/space each.
If you can confirm the following I'll knock up a couple of box designs this week.
Amp model number
voice coil configurations (i.e. ahve you got a dual 4 ohm voicecoils on your subs?)
boot space
If you just want the recommended box sizes:
http://mobile.jlaudio.com/pdfs/15_18W6_BDS.pdf
