1 1000. I can see 2 600s getting kinda tight in a 4x4. You would need 2 Y connectors and stronger fan to cool and filter them, and that's assuming that you would do it that way. If you did it independtly, you're talking two fans, two filters, etc. It would end up costing you a lot more money for 200 more watts. You should get monter buds in a 4x4 with a 1000w if you do it right. With the 600s you're going to have to double up on AT ALEAST bulbs,ballasts,hoods, fans, and ducting.... more heat (especially if they're not digital, although those get warm too).