400 watt heat concerns

I just bought a 400 watt mh, hps setup... is a 4x4x5 closet to small?

to cool your grow room use circulating fans to cool plants. Exhaust fans to stop heat buildup in grow room and a/c to stop heat buildup in your grow building. Cracking windows is another option. Higher CFM exhaust can also create negative pressure that will cause the entire grow room to recycle fast enough that there is a breeze to cool the plants.

clip on fans can help expedite the heat to the exhaust port if you clip on to the open end of the hps reflector. also there are reflectors and cooltubes made for dealing with heat.

with 5 foot vert and all that horizontal space I would run my bulb vertical to maximize efficiency. Use a box fan under the bulb to help disperse the heat away from the plants.