A good place to aim is 50w/sq ft, you have 10.5sq ft and 400w = 38.1w/sq ft so your not far off. Your plants will grow ok with that light, if you already have a 400w. If you are looking at buying a light I would suggest a 600w. That would give you 57.1w/sq ft, much better!
Whichever sized light you use, look into cooling it somehow, with a DIY cooltube made from a bake a round if your on a budget or a proper air cooled hood if you're feeling flush. Doing this would allow the light to be closer to the tops of the plants, giving them more lumens and growing fatter buds, at the same time as making the most of whatever height you have in your room.