Having posted hard air moving facts....I looked into it further.
I delved into this very deeply. While a 6 inch vent pipe is not rated for more than 100 CFM....a 4 inch vent will move adequate air (even with filtering)....with a couple caveats.
You really only need a piece of the 'normal' blue furnace filter....use a generic 'flapper' type outlet rather than a vaned outlet...keep your vent pipe as short a possible...and start your fan a couple minutes before you begin painting.
The coefficient of friction for moving air is much different between 6" and 4" venting. 4" should move all your air if you jump start it and get things flowing. AND not restricting the outlet.
I am very sorry if I lead anyone astray on this point.