Definitely go with the 4cubic foot box. Tune around 35hz, it will get VERY loud and should still sound decent enough for daily driving. If you build yourself, you definitely will have to brace the hell out of that box.

 

Thanks for the info. i have never tried a ported box becouse i thought the sub would hit bottom even more. but what do i know, you don't know untill you try it right

 

Not fameliar with the ride, is it a hatchback or sedan coupe ?
And I agree The cabinet will make or break the sound.
I'd also suggest that if SQ and control of bottom end is what you're after, go for a higher impedance, it may not give you surrealistic bass levels, but it'll certainly blend well with the comps, but that's just IMO.