I am trying to hook a bathroom vent pipe to a roof pipe vent jack. Last Summer I had the roofing set up a vent for the bathroom exhaust. He put in what looks like a code cap and base. At the time there was no bathroom exhaust vent. Now, I am trying to install it and the plywood is cut out for a 4" duct ( like it should be) but the code base looks like it is for a 6" vent pipe. Is there anu adapters out there that are a 6" vent ouside diameter and have a 4" vent pipe in the inside. This way I could seal the pipe at the code base, but still use the plywood hole that is already cut out in the plywood. Any ideas. I will attempt to attach a picture.