I have a 400 in a 3x4 space. It works well. But you can tell the difference between my buds and those grown under a 1000 watter. Those buds are much more dense. A 600 is really a good compromise, but more watts mean more heat. In a small space like 3x4 or 3x3.5 you will need a good fan on your light. I have a 171cfm on my 400 in a breadtube,cooltube. It works very well, exepct in the summer, there should be a 250 on it. That being said If you get a 600, make sure you get a fan that will move enough air to keep it cool. Of course the same goes with the 400, you just dont need as big a fan.