here are the pics of the ducting i posted last season.Magnum stoker settings for the control box
i connected the 6" from the mag to my existing 12" X 14" registers which have variable speed refrigeration condenser motors in them. the variable speed blowers increase the velocity of the stove air upstairs, plus also pull air from the basement ceiling. the variable speed allows me to keep the 2 floors the same temp, and works very well & quiet. i blocked 1/2 of the dist. air coming out of the front of the stove with bar magnets right in front of the 6" take-off, so some air is still blowing in the basement where my older son's room is.
why are you going to restrict the flow to 4" upstairs? i was a sheet metal mechanic in east orange, nj for 20 years & we would only come off the main with a smaller run to feed a bathroom or small utility room. go with the 6" upstairs (2 of them) and you'll be pleasantly surprised. the transition from 6" to 4" will only restrict the airflow & cause more air to come out the front of the stove. you'll be increasing the velocity but the volume will remain the same overall. just my 2 cents.
i'll keep you informed of the 'outside air' thing when i get it hooked up later in the week. i do feel a slight draft coming under the door leading to the garage in the basement...i suspect that will stop once i get this done. as far as the colder/denser air being better for the burn???...i dunno? it gives better lift for aircraft as opposed to hot/humid air...but my mag stays on the ground. i wish someone knew more about that.