my area is about 2 x 3 and 400 just barely cuts the cake...
but i also have a really old ballast with a conversion bulb that runs at 360 actually.. but 400 w/ a 3 x 5 area will work... for a start IMO...
based on if your flowering I just realized....
For the vegging your ok the 400w MH will due fine, but for flowering you need to be pushing 5000lumens per sq. ft. at a minimum which a 400w will not provide u. your around 1000 or so lumens per sq. ft. short to be at the minimum for flowering.