Do-It-Yourself, Repairs and Fixes
Share tips and tricks to keep people from throwing out that broken item. Repair before replace!
Without pictures, a best answer is not possible.
I'm going to rely on experience and assume that there is a 90° bend elbow in the wall and the last segment has broken away.
With the original piece you can open half of the flange and slip the removed piece back in and fold the flange back over.
If you don't have the original piece then you can get a "4in flexible duct connector" and slip that in carefully.
You can also slip the end of the flexible duct into the vent, but you don't want to bunch it up in the wall, you want the flexible duct to have as smooth a wall as possible.
I don't go over there often, but I'll check next time I do. The duct connector looks like it usually goes over the duct/pipes, so maybe it would be difficult to insert into the wall duct? I could always try, though.
The flexible duct does normally slip over the vent, you can insert it with some doing.
If it is the silvery plastic with the metal wire inside, that is easiest to slip in. If it is the corrugated aluminum duct, that will require crimping with needle nose pliers to shrink the diameter.
If the hole is round, I’d look for a piece of PVC with an outer diameter that matches the inner diameter of the vent pipe. Use it to connect another piece of vent pipe that is the same size as the broken vent pipe. Glued in place it should work just fine.
You are probably looking for something like a 'Dryer Wall Flange": https://www.homedepot.com/p/Everbilt-4-in-Dryer-Duct-to-Wall-Connector-EZDOCKHD/205046832
That would work if the vent were straight, most likely it is a bend. That could still work but the flange would need trimming to mate up properly and a high-heat sealant would be needed to ensure the lint doesn't just get blown into the wall.