Will you be exahausting the hoods that hold the 400watters..?
If so..... you should keep the light at 6-12 inches... from your canopy....at the very very most..

400 watt bulbs produce signifigantly smaller, less dense buds than 1000watt bulbs... but you can over come this a bit if you place the lights near one another.. and keep the plants short... especially if you are growing many mnay plants.. Flower when they are about 12 inches...

I would ike to see a picture of your grow if possible..

cheers