The real answer is to duct hot air from your light somewhere else. If you can do this you'll be golden since most of the heat comes from the light.
Unless your in a cold basement, or the room stays less then 65 degrees, a 400w light will heat up a small room with the closet door open. Especially when it starts getting hotter outside. That light is like a radiant heater, you can move all the air you want, if your not exhuasting it somewhere else it will just get hotter, and hotter, and hotter...unless it's the dead of summer and it won't get any hotter outside, expect that 80 to reach 90+
I have a 600w light in a cold back room in my basement that is approx 9'x25' feet. With the light off it get's down to 60-65degrees in the dead of summer. During 90degree days with my light on I have a hard time keeping it below 80 degrees. I have an air conditioner I use just for these hot days. The point is I have an ideal grow room-in a cold unfinished part of my basement-that a 600w heats up pretty good. I use an air cooled hood in a 4x4 tent, but blasting the hot air into the room heats it up pretty quick. I rarely have to use the a/c, but I do sometimes.
If you can get an air cooled hood and duct the air outside you'll be golden. The hardest thing about a closet grow is heat control and air exchange. You can grow in the closet but your gonna need all sorts of ducting coming in and going out.
