Yess im using a 6in inline duct fan, im sure a 4in would work cuz I rarely use it on full blast, Having a friend who's dad owns HVAC store kinda helped me out, thats all he had so thats whay I got. I have prob less than 80 bucks in my CAB. Im not here to tell anyone what to do or if their ideas are wrong cuz mine all started with a idea anx it works great!
Back to the carbon filter, the reason I didnt mount mine onto the fan was cuz that was alot of weight, I ran 6in duct to the floor where my carbon filter sits, all air from onsode the cab comes out into the filter, have not ever had a problem with the smell.