Does not fit a 4 inch pipe - it exactly butts-up to the other 4 inch duct - not into or over it.  I bought this item to reduce the 4 inch hole on the PSI Woodworking DBGULP Big Gulp Dust Hood I bought for my table saw. I have had to make so many "adjustments" with duct tape to make my dust collection system operational. This fits exactly nothing and the industry seems to need to look up the word "adaptor" and "reducer" as this should fit either inside or outside a 4 inch hole (industry pick one and outside might be a better choice). I have run 2.5 inch pvc through my garage and have several of my dust makers installed (again duct tape to snug the fit in almost all cases). In my garage I don't need or want a 4 inch pipe.  All that said I blame myself for not looking at all the reviews however, the words reducer are grossly misused.  Providing measurements might have helped some.  It is sturdy hard plastic.