I will be using a cool tube/air cooled unit with two 6" inline fans. Is a 600w MH/HPS too much 4 3'x3.5' area? Height is not an issue.
That space is about right for a 400w, but I agree with FunkDoc, if you can keep the heat under control, and don't mind the 50% higher power bill, go with the 600w.
Does it still hurt the electrical bill that much if it's a digital ballast? I heard those are 30-40% better than regular ballasts.
I always see questions asking if there's enough light. I just don't want to over doit but the air cooled tube reflector with two 6" inline fans should be enough to keep it cool. I might have another 6" inline fan to bring in fresh air and of course one to circulate air in the room but the fresh air fan is a maybe at this point (another hole in the wall).
Ain't no such thing as overkill, however, you might want to consider using a couple of cfl's for your first couple of weeks to hold down the elec bill. p.s. I got 2 400s in a 3.5x5 space and it could use more.
A 600w lamp with a digital ballast should still use 50% more juice than a 400w lamp with a digital ballast...
If that's the case I think I will go with the 400w, I just want to have enough light for about 6 plants in that space.