Ventilation

How much ventilation would a 4x2x10 closet grow with 4 plants need? Ive got a standing fan going already blowing on the plants and the door is always slightly open. I am wondering if I am going to need to purchase any exhaust pumps or anything like that. Also, I am using worm castings for fert., will this be enough to sustain the entire grow? Would it be a good or bad idea to mix chemical fertilizer with organics?