400w is a lot of heat to dissipate out of a closed box.
No, it's not. Lots of servers draw 400w in a 1U case. 4U cases can handle well over 1000w, easily.
Ok, fair point. I would argue that those 1U cases and the components within are specifically designed to withdraw that heat in a particular way, with hundreds or thousands of engineering hours dedicated to exactly that, but it does show it is possible.