I'm not worried about the fan. It will be a 4" inline fan readily available. My concern is forcing it down to 2" where it meets the PVC. I have found a 4" to 2" reducer but it isn't actually a reducer in the true sense. It is a 4" to 2" PVC bushing and it doesn't reduce - it's an sharp angled reduction (90 degrees). This is going to be a custom job to be sure.

I've refined my rig config a bit:

charcoal scrubber > 4" exhaust duct > Light hood > 4" exhaust duct > inline fan > 4" exhaust duct > damper/draft control > step down to 2" exhaust PVC > Y connector with shared vacuum line > vents outside.

I'm guessing that I will need a speed control unit and that it will be a question of finding the optimal fan speed. Appreciate any help from the board before I go buying components. This is my only hope for external vent.