Product review (2 stars): 4" vent
This will fit a 4" vent pipe. After buying it, we realized the hole already cut in the wood for our vent to the outside is 6".  The vent just fits over a 4" duct but not tight enough to hold it in place if the pipe isn't secured to the wall somehow.

To the guy that claims this is UV resistant plastic- the instructions included tell you to paint it for UV protection. And yes, the screw holes are indeed very close the corners.