Product review (1 stars): but good luck using the "gasket" that comes with it
The others are right about this not fitting a standard 4'' duct, not even close, i can jam the plastic onto the crimped end of a 4'' tube that is crimped even a little extra to make it a little smaller.......but good luck using the "gasket" that comes with it.  If I were to have used it, I would have had to use aluminum duct tape to fasten/seal it..............and then what is the point?, and the other side is very difficult, like almost impossible to get onto semi rigid flexible aluminum ducting.  Nice idea, terrible execution.  I threw it in the trash, not worth the time to send back and pay shipping to do so.  If you are looking for a solution to make connecting your ducting easier, this is not it.