Is 4ft. sufficient for a 400W 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?

drop it a few inches every day till they react to it, but 10" or so above sounds perfect (I'd try it lower than that), 4ft is too high (unless they are little seedlings).