i got a 400 watt hps and 7 cfls in my box. the minimum bulb that i would recommend is the 23 watt...equivialant to 100 watt incadescent. make sure they are 2700 k warm white. those are what i use, they are cheap and readily available. there are bigger, but with a 400 already, is there really a need to invest in expensive cfl's.
4x4 is pretty big for a 400, mine is 3.5x3.5 and i feel a little outsized. so i would def. put in some cfls. i think that the target lumens per sq ft. is 7000-10,000. you have 16 sq ft, and lets say that the bulb puts out 50,000 lumens. this puts you at 3125 lumens per sq. ft.
this being said there are alot of other factors, such as light distance and so on that go into the lumens per sq ft., so dont stick to this figure like the bible. you'll be fine with a 400 and some cfl's in your space. especially since you only want 3-4 plants that will fit directly under the bulb. it is just if some day you wnat to pack that space, there might be some issues. good luck!
