I had this problem when i first got my 1000 watt light to I found when I step up the wattage the heat goes up by several folds, My plants was not use to the that much light and it started to burn them. I posted on this board and someone else had same problem they told me to move the light away from the plants as far as you can go and let them get use to it and slow over time maybe week to ten days lower light a little at a time. Keep in mind you will not beable to get as close to plants as your 400 watt light. If it was me I would take the 400 watt mh light out of there and use in veg state on up coming plants. Hope this help Bizzy.