How do you plan on building it? Make sure it is PYREX or it could shatter from getting too hot. And no 4 inches is not long enough, im not sure exactly but my 400w hps is probably around 8-10 inchs long. Cool tubes dont have power so how would it need to be wired to the ballast? Its just a tube with a fan on one or two ends and some ducting to remove the air from the grow room/cabinet.