I was able to come up with a very easy way to vent a 4" exhaust hose out a window using a $3.18 part from the hardware store (4″ snap in drain). It is the perfect size to slip a 4" hose over the top, has a nice flange for mounting in a board in your window, etc…
I actually used an adjustable aluminum 90 degree elbow to go between the flexible hose and the snap in drain, which makes it very easy to push on and pull off when not using the glowforge.
I also bought a 4″ PVC DWV to Sewer and Drain Bushing ($2.48), which fits over the outside of the snap in drain very tightly. I hot glued a piece of foam inside it to block drafts, so when the hose isn’t hooked up, I just slip the bushing & foam plug over the plastic drain to seal up the hole.
Photos and a full writeup are here:
For anybody worried about the fact that the same board is used for the inlet and exhaust of my 2 hose portable AC unit (and the exhaust vent is in the same window opening as the AC inlet…):
The portable AC unit draws air from the outside on the right side of the window, uses it to cool the internal condenser, and then exhausts it back out (as hot air) out the middle opening. So any glowforge exhaust going out the left hole would have to make it past the blowing AC exhaust from the middle hole before it could get sucked into the AC intake on the far right.
Even if some glowforge exhaust did get sucked into the AC inlet, it doesn’t go back into the room, it’s just used to cool the condenser and then exhausted back outside again. (And I doubt a little bit of plywood / acrylic smoke is going to harm my AC unit’s condenser more than the normal outside dust it pulls in on a regular basis…)