76 cubic feet needs cooling and scrubbing

I'm looking at htg's 4" growbright/filter package. It's cheaper than the 6". What do you think? Open air 400mh now, 400hps later. Got mylar this time to save the lumens and it's helping to warm things up. Starting with seven 3.5 (or so) gallon kitty litter pots. Used a 4" duct fan last time drawing through a crappy diy filter--two plants stunk up the house.

Please give me some advice.

i use the 6inch setup and it works well.
your space being 76 cubic feet is about right for the 4inch fan and filter. my 6 inch fan is a bit loud tho to i had to isolate it to hide the noise so i'll bet the 4 inch fan will be a bit louder then u think