It depends on what kind of lighting you put in there and how much heat it creates as to what size ducting, fan and carbon filter you use. If you're using lighting that doesn't put off much heat, then a 4" setup would work fine. But if you're using MH/HPS, you're probably going to be better off going with 6" and a high output inline fan. Heat is always the issue in enclosed spaces. TWW
Agree with widow. Since your using LED I would say a 4 inch will work. A 6 inch would still be better though.