4 foot would be a safe distance, but could stretch the seedlings making them weak-legged...

like stated in other replies...get light as close as possible to canopy with a constant fan blowing between the light and the canopy, without melting them...a good test. After you set your light, let's say, 18" above the canopy...place your hand right at the top of your canopy, and if your hand does not start to burn after a minute; You are OK. check often, or even back your lamp off a 2-4" if you "feel" that your lamp is too close. hope this helps. lw

Make sure you have a nice little fan that blows cool air between the lamp and the plants.