Dude, 10sq ft is plenty of space for 4 plants, but the heating issue w/ hps and mh bulbs will require superb ventilation. You may want to consider fluorescents, there are a ton out there and they use less energy and produce less heat, but the penetration of light is less. And for the mylar film, you should line the whole box w/ it, or another alternative would be painting the inside with white reflective paint