I'd much rather go over in Lumens, than under...

4x4= 16sqftx5,000L = 80,000Lumens needed.

-Most 600w HPS = 88,000 - 90,000 Lumens
-Most 750W HPS = 110,000 Lumens
-Most 1000W HPS = 130,000 - 140,000 Lumens

So IMHO, I'd go with the 600W as long as my room was a closed in 4x4.