400w primary grow area is what 4'x4'. I think they are going to be battling for light assuming tthat those bucket are going to grow some monster girls and your going to want them spread out a bit for air flow.
I did 4 plants in 3 gallon pots under my 400 watt HID, and they were kind of crowded when I flowered at 12 inches. I will do it again but flower at 8 - 10 inches next time.
like wolf an ian said veg to 8-12". i did 3 in 3gal pots under a 400watter in a 2.5x3.5 room, veged them to 14" and it was to much, thay ended up over 5ft tall with no air flow.