few questions? how big is and what type of grow space do you have (closet/tent/stealth/etc.). Which 4inch fan do you have? what hood do you have? do you have a bottom cover for your hood? do you have ducting that fits your fan, hood and filter?
you can use a 4 inch fan with a carbon filter. I wouldn't have it running thru a bunch of tubing and a hood as well thou! A 4 inch fan would be bogged down by the bends in the ducting, going thru the hood and trying to pass thru the filter! If you only have the one light and the one fan I would question what i needed more the smell gone or the heat removed from the hood! I would start by hooking the exhaust fan up with the filter and checking to see if this does a good enough job to keep the room cool if not then hook the fan and ducting to the light. do you have a glass bottom for your hood? hooking an exhaust fan to the hood with out the bottom cover doesn't do all that much because it doesnt draw as much air over the light. I have seen plenty of setups where the filter is stuck on one end of the hood and the fan and ducting are on the other, so it is sucking out the smell and the heat at the same time. but I have also seen a few people try this with out the bottom cover on their hoods and wondered why the smell wasn't going away! the only fans i have for my lights is are oscillating fans. besides that the heat is suck out with the smell thru the carbon filters. (I also closed off the heating and a/c ducting that went to the bathrooms and added them to the grow room so when the a/c comes on that room gets twice the cool air!)