this seem to be the argument for sealed room growers, when and if to exchange air... if your supplying the co2, air con and de humidifyer then there seems to be no reason to exchange the air. Some argue this but if you do it right you should have a closed environment that can regulate its self producing its own o2 for its root system.
You really do need the space for that air flow and control over the temps and RH, maybe get away with it in a 8x4... I have dream , a shed of about 12x16 fully sealed