Either set-up would work. Using just four 1000-watt lamps is convenient and will provide good light coverage with the right reflectors. Using five 400-watt lamps would cover the space more evenly so that growth would be more uniform over the entire area.
Air or water-cooled reflectors eliminate most of the heat created by the lamps, so the main consideration when figuring the distance between the lamps and the tops of the plants depends on the type of reflector being used, not the heat factor. In the best configuration, the entire garden is well and evenly lit, with as little light escaping to the sides as possible. In some instances, reflectors can redirect the light back to the garden.
If you are using 1000-watt lamps, a light mover that moves the lights back and forth over a two-foot area (rather than the usual six or eight feet) would even out light distribution.
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