Well, wanting circa 50w per ft^2, the 400 would illuminate 8ft^2 (which is exactly your 2'x4' space. The 400w would be perfect for your setup, you will just need to make sure that your area is properly exhausted because a 400w light will heat your 48ft^3 room quickly.

The other thing (if you're open to suggestion) you should do, is cut back the number of plants. More plants does not mean more yield, and 48 plants in that small of an area is really overkill. My buddy runs a 600w over a 3ftx4ft area and flowers only 24 clones in that space. If you grow your clones a little larger, you can dramatically cut back their overall numbers.