a 400 can sufficiently light a 2 by 4 area if it is positioned right there is plenty of cabs that are 2 by 4 with a 400 that had great results however they were all scrog wich was why they yielded so good 2 400's is right at 50 watts per sq ft.
the most you should use is 1200 watts whether it be 3 400's or 2 600's wich is 75 watts per sq ft and if you dont intend to do a scrog 2 600's would probably be a better choice.