To utilize a 400W HID in a 2'x2' tent properly and efficiently your going to NEED some kind of air cooled hood or cool tube with ducting and at least a 4" inline fan. A 250W HID on the other hand could be used in an open reflector with just a fan blowing on it, however you will need to vent some of that heat out of the tent with some kind of fan like a computer fan, ideally you will have an intake down low and exhaust up top. This is just recommendations based on research Ive done and people Ive talked to as I'm no experienced grower, yet.