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