oh, idk.. when people state their cu ft, its widely accepted that, that is their net cu ft. net cu ft is after port, sub displacement, and bracing is taken into account..
so when you say 4 cu ft, that means your box, with subs, bracing and port is 4 cu ft..
you will probably try a few boxes, everyone does. you can try 3.5 cu ft, 3.25 cu ft, 3 cu ft.