it is a general rule to try to have about 50 watts per sq ft of hid lighting for flowering per example 8 sq ft divided into 400 is 50 watts per sq ft.
take your width times your depth of the cab to determine sq ft and see what size light you will need to use, a 2 by 4 ft cab would be 8 sq ft and is plenty of room for 4-6 small plants.
you will need some decent ventilation to keep temps stable and would build the cab tall enough to have plenty of room probably 4ft tall at the minimum for a 400 possibly taller depending on size of light and growing meduim like pot size or hydro system size.