Check Ventilation 101
and the The 400 Watt Club
The chart below is for cooling only. Adding a scrubber will add resistance lowering fans effectiveness
4x4 may sound small and everyone smokes their own way but, my grow is barely larger than 2x2 under a 250 and I can't begin to keep up with it's production. Hell, I've even done some jar curing lately because I can afford to lock weed in a jar for 6 months.
You only need
4 plants if you grow 4 strains. It will take longer but, with LST
, 1 plant will fill your cab.