I didnt even read the whole thread so maybe someone mentioned this already but use a 600 digital HPS ballast. They produce the heat of a 600 watt with nearly the lumens of a 1000 watt. They also can run both HPS and MH bulbs (although they will degrade the MH bulbs faster then a normal magnetic MH system). The ballasts themselves also stay cooler, so if your ballast must be in the room then every bit helps.

That being said, a 6" fan on one 600 watt lamp is going to be MORE then enough. Thats a lot of CFM. Ive run a normal 1000 watt HPS in a room smaller then yours without much problem (except on the really hot days which is where you need 2 fans, one sealed for the light and one for the room). For a room your size, to take full advantage I wouldn't run anything LESS then a 600 digital. A light with 100,000+ lumens, for best results, should really not be used on anything larger then a 3x3, but 4x4 is of course do-able. Not saying you cant get good results with a 400...but atleast make sure its a 400 digital ballast, which produces the lumens of a normal magnetic 600 watt ballast.

For reference, I run a single 6" fan to cool all 4 of my 750 digital ballasts. Granted its sealed, but gives you an idea. In my 5'x7'x5' grow box I run 2 600 digital lights with a single 6" fan, this time pulling air from the grow room like you want to. I have to use a speed controller or my temps get to low. Thats a 175 cubic foot room, yours is not all that much bigger. I like to have a CFM that is bigger then the room incase things get really hot or humid. I use an 1100cfm fan on my 1000cubic feet grow room. It can completely exhaust CO2 and stale air in a little more then a minute. This allows me to only run the fan on short intermittant bursts. Saves energy and other things.