Happy...

Take into the formula....

a: the hight of your growing container.
b: the light must hang from the top so how close can you get it?
c: the plant canopy must be a certain distance from the light source.
d: a 400W HPS gets hot as fuck...increasing the need for space from light.

Instead of a SCROG you may want to consider and LST....

But take those factors into consideration when choosing.

As far as heat, you need:

a: a fan blowing on the bulb itself.
b: a powerful air extractor removing the hot air OUT of the grow area.

A 250W HPS will work fine for a 2x2 area.

Good Luck