Why? Do you have a 4" cold air kit and a 3.5" MAF? If so, you can get 3.5" ID tubing from McMaster Carr and cut 2 pieces to about an inch in length. Put them on each end of the MAF and the 4" tubing will go over top of it nicely since it's close to 4" OD.
If that's not why you're asking, please disregard.