Eh how much power are you running to each sub?
Whenever you are shrinking boxes you'll lose your frequincy range, mainly lows
now I know this is not always but I am refering to shrinking below recommended sizes by a great deal.
I would not recomend making your sealed box into a ported one, we could try a ported box with two of your 12s? But 4 12s in a single cab is alot, ported isn't impossible but I don't think it would be worth what you would have to give up
example being leg room or your seats tilt back.
Also I don't like to build boxes that cram passangers, I could, but if no one but a midget can sit in your seat I'd imagine you would be keeping the box for very long.
So if you are set on having 4 12s ported I would recomend cutting something out.
Also if you would try to port your current box you would have a very short frequnciy(sp) range you could play and it would be higher freqs at that, so not very injoyable, well to me that is I like lower tuned boxes for daily ie 28hz or so