rocket mass heater: a smaller hole in the wall

Since the exhaust from an RMH is just a tiny dribble .....

And since the 8 inch myrtle system drops to seven inch ....

I wonder ....  if I have a six inch system and then I get to the last little bit of exhaust that goes out the wall like a clothes dryer vent .... and a clothes dryer vent is typically 4 inches ....  would it be okay for the six inch system to reduce to a four inch system at the point it goes through the wall to the great outdoors?

you can just make sure the exhaust temp is low enough for that to work or you will have smoke back problems