I second or third the 600W vote, if you can keep the temps right. Unless you thought you could add another 400 some later date but I don't have tent to know if that's possible. I have 2x400 in 4x4 area and think 400 wouldn't cut it, certainly not for the buds. I know your only vegging but you probably still want some nice sturdy stems and I feel the plants around the perimeter would be flimsy with a 400.