Ok, well, I would get a small charcoal filter, or, you can make one...
hxxp://boards.cannabis.com/growroom-setup/90173-how-carbon-filter.html

However I think realistically, you will need a more powerful fan, since a filter produces a lot of 'drag' or resistance, because it has to pump the air thru the filter. It would also help if you went from 4 in to 6inch venting, and if you insist, bring it down to 4" just before bringing through the wall. Its always best to use metal pipes which are smoothe on the inside for better flow, and keep the line as straight as possible with as few bends as you can. Air goes best without resistance, and the flexible pipe is not near as efficient.

formerly in HVAC biz for 10 years.
hope this helps
SpaceNeedle
SpaceNeedle Reviewed by SpaceNeedle on . Need Help with Filter Selection -pics new grow I am in week 5 with my new setup. DR100 Grow tent 3 x 3 ebb and flow table 400watt vented MH. I love this setup but I think I need a charcoal filter. I have a 4 inch windtunnel fan 200 cfm mounted on the ceiling above the tent.Exhuast out the top of the tent with about 12 feet of 4inch ducting through the wall and exits as a dryer vent outside. Even though still in veg the sweet smell is definately noticeable on my back deck. I live in the woods but still dont want someone wandering buy and Rating: 5