Is 4 ft. sufficient to grow under a 400W MH light?

Here's the scenario, I have a closet that is 4x3ft and the height is about 8ft. So I splitted the closet into two compartments, giving each about 4ft of height. Each of the compartments will have a 400W metal halide. The bottom has 6 plants right now and the top I will plant to put 8 plants when I make the clones. My question is, is this sufficient height so that my light isn't too close to the plant that it will cause burning of the leaves? I have one exhaust vent but no intake vent, just a hole at the bottom which, hopefully, will passive air in. I dont think of putting another exhaust fan in unless heat becomes an issue. Any takers?