Running it down a wall cavity isn't very efficient (hot air rises). You might be able to run it out the wall under the sofit by just running down the bottom of the ceiling joist bay. But, so you don't fight the tendency of the hot air to rise, running it out a roof vent works better - probably a shorter run and works good. I'd run the 4" and forget the reducer. Panasonic is a good brand, so it probably wouldn't cause vibrations, but you might end up using a little more electricity, it'll be a little noisier (you may not notice), and it won't move as much air.