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The 400 MH is more than enough for veg, I use a 90 LED for the same area.
600 Watts HPS is the minimum for 4' x 3'. Sun System has a great light foot print for a rectangle, I have the Sun System II.
A light mover would benefit even in that small area, 1' - 2' travel: changing the angle and hitting different light receptors; reducing heat stress; high light concentration over different parts of the garden; etc.
If you have a second timer put your 400 W MH on for 4 hours midday, like the bright sun around noon. The plants don't use 12 hours of continuous light effectively, a boost through noon will be like raising their metabolism exercising for an hour and it stays higher all day long.
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