William Duffield
Maybe something like this could be adapted. You would have to cut off the 4" end.

https://www.woodcraft.com/products/4-x-2-1-4-funnel-reducer-dust-collection-fitting

They also carry one with a 2" i.d. The best strategy is to carry the part that needs to fit it to your nearest Woodcraft (or Rockler, or Klingspor) store. They all carry a wide range of dust collector adaptors. Another place to search is the plumbing department at your local orange or blue borg.

But, as Dick Cours suggests, I think I would try to turn one to fit, since that would be the most fun solution to the problem.

Or, what about some duct tape?