Now for a 4x4 space i will have to agree with fat sam. Your most efficent setup would be running a 600 hps in each tent. If your space was a 3x3 a 400 would work, but a 600 will always be better. More light the better.

You will see faster growth and more yield with a 600 hps then a 400.

Good luck on your grow and keep us updated