Personally I dont even see how you could fit 4 cubes sealed, let alone ported, and still have a usable back seat.

To give you an idea of how much space a port can take up, I figured up my buddies box yesterday, and the port and driver displacement took up .7ish cu ft, so thats a box that has the same outside dimensions as a sealed 2.7 cu ft. box...and its a 2 cu ft box.

How much airspace do you have for each sub right now?

Personally, if you can accomplish it, I would make the box all one piece if you can, no 2-piece.

 

something a lot easier would be to get a sub that belongs in a sealed box!.....

 

all speakers work both ways and almost all perform better in a tune ported box. Some perform leaps and bounds better in a ported box. Besides I don't think I'll live through the wife if I ordered a pair of mags.