I agree with kangaroo, 2 plants in 20-25 gal is about right in a 4x4 and a 600w should handle that well. I think 4 plants in 20-25 gal in a 4x4 is too tight. And your 600w light will be better in a 4x4 than a 5x5. If you really want to do 4 plants, scale your pot size down. I did living soil...