The 400w = 400g is based on the assumption that a good grower can get 1g/w out of a grow, but this is by no means a maximum, and is not necessarily achievable, depending on the strain..
I assume those measurements were in mm, which would give roughly 25sq ft, more than enough space to grow 10 plants. A 400w will cover an area between 2.5x2.5' to 3x3', and you might just squeeze 10 plants in there, but you will need to flower quite early to stop them getting too big and shading each other.
The space won't affect the yield, it's the amount of light you put in there. General consensus is that a 1000w will cover a 4x4' primary growing area, with an extra foot around the edge of secondary lighting.
I would suggest getting the biggest light you can afford to run and keep cool. I have a similar sized space as my flower room, but I only grow in a 3x3 area under a 600w, with 6-8 plants at a time doing a perpetual grow. I usually get 40-50g per plant, and harvest 2 every fortnight.

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Originally Posted by ceestyle
Don't be a martyr wasting $100 or more on CFLs to prove they're inferior for flowering pot.