When you need to change from a 4” hose diameter to a fitting accepting a 2-1/4” shop vacuum hose, this fitting offers a long tapered transition.
The outside diameters of the ends are 4” and 2-1/4” shop vacuum size. Just push the 4” hose onto the fitting and clamp in place.
The small end matches the size of a 2-1/4” shop vacuum so you can now connect to the many router fences, bench top sanders and more that accept this size.