AIRFLOW!!!!
it makes all the difference in the world. with a 400 watt you can get pretty damn close to a plant contrary to popular belief. Its all about movement of the heat. I keep my 600's roughly 6inches away from tops at times ..keep in mind i have very good airflow in my room with air cooled hoods that allows this closeness. There was a gardener on here that used a 1000 watt about 8 inches to a foot away for CLONES, jus gotta have the air/heat issues taken care of. Then theres lumen disbursment...if your plants top is that close, then its soaking up most of the light you know...To answer your direct question, given that you have your reccomendened light per square foot, and air/heat movement up to par...your plants will not get burnt 8 inches away from a 400 watt hps bulb.