In a set up that small, you might be better off doing a really slow constant release of co2 with the emitters really close to the plant so its getting pure co2. by doing a really slow release, you conserve co2 and no longer have to worry about vent fans.

If you go the other way and want to saturate the room in co2 you are going to waste a lot of it through air exchange. since a 4x4x8 area is very quickly heated by a 600HPS, your fans will remove much of the co2 before it is used by the plant(s).