Ventilation Question for my Cab...400W HPS

you will definately want to get an air cooled hood or a cooltube.
i may be best to have to exhaust fans.
-have a an intake duct from outside connected to one side of the cooltube and out of the cooltube to the exhaust then back to outside.
- have a second exhaust pulling air out of the room from the ceiling.

i use only 1 exhaust and this is what i used. it is a 6" to 4" reducer. drilled large holes in it. connected to the 6" exhaust duct and the reducer is up at the ceiling. then i ran a 4" duct connected to the cooltube that i made. once i tested it, i put duct tabe over a couple of holes in the reducer to increase air flow to the cooltube or remove tape to decrease flow. i only ended up taping 1 hole. the holes in the reducer still take heat away from the ceiling and still cool the light. hope this helps. and i use a stanley blower for my exhaust. and you will need at least a 7" or 8" passive intake.