I too run a 4" filter and 4" fan combo on a variable speed control...(bought as a matched set )...been using it 1.5 yrs with no prob in a small tent. Ya know just how good they work when ya pull ya ladies out to dress'em a bit and the house gets really stinky in a short time.
At the cost of 'bought' filters, I would have to agree that I wouldn't take a chance...When I'm out for a day, the first thing I do when I walk in the house is 'sniff'....