good point 4x4 room is like right in the middle too big for a 400 to small for a 600 well not too small but you could go a little bigger with a 600I think that you will find that a 600w would do better in that space so long as you are air-cooled. I bet that if you get a 400 you will be wishing you had gotten the 600 sometime not too far down the line, so I would just go straight for the 600. It happens to everyone. In a 4x4 with an 8 in. exhaust you should be able to keep it cool enough with a 600. other than that it seems like you're on the right track.