With a 400 i would imagine the temperature in the closet would stay around 10 degrees over ambient air temperature.

I dont think twin 150's would really heat it up at all, a degree or two over ambient.

Keep in mind as you go up in HID bulb wattage the lumen output increases by magnitudes.

I wouldnt be surprised if the output from 4 150's was lower than the output of one 400.

In terms of footprint, put up some white plastic or mylar, and the light will be more than adequate. You said your room was 3'x5'. Your average 400w light can cover 4x4ish (dont expect huge yields). Rotate your plants every few days in the closet so every plant gets good light.

If you can vent the closet and duct the light you could easily run a 600.