The Idea is to get 10,000 lumens to the plants.
A 1000w HPS @ 4ft=10,000 lumens
a 600w HPS @ 3ft=10,000 lumens
a 400w HPS @ 2.25 ft = 10,000 lumens (taken from Gardening Indoors)
Using more lower wattage lamps is more efficient in lighting and per Kw cost however is more expensive to set...