To avoid any problems and maintain a proper temperature in an enclosed area you pretty much must have a Can-Fan or Flow-thru fan of some sort to remove the heat that is put off by the light. The only time you can really get away with not using a fan or ventilation in a grow room is if the light is an open space where the heat won't stay around the plant but disperse into the surrounding room.

400watts really isn't much, it's about equal or even less than the average computer power supply. The big issue with HiD lighting is the heat...

hope this helps,
Bob.