If I was you, and if you never ever want to have to worry about even the slightest odor, then get a 4" flange filter with at least 11lbs of carbon.
Dutch breeze 4" with 11.9lbs of carbon is like $90 of BGHhydro.com, you need a 4" vortex fan, but you will not regret it. You will never have an odor issue, and you will be able to run some 4" ducting to exhaust the closet and get better airflow which means bigger buds.