CompatHelper: bump compat for AbstractMCMC to 4, (keep existing compat) (#61)

This pull request changes the compat entry for the `AbstractMCMC` package from `2, 3` to `2, 3, 4`.
This keeps the compat entries for earlier versions.



Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

Author: github-actions[bot]

Project.toml

@@ -12,7 +12,7 @@ Setfield = "efcf1570-3423-57d1-acb7-fd33fddbac46"
 SparseArrays = "2f01184e-e22b-5df5-ae63-d93ebab69eaf"
 
 [compat]
-AbstractMCMC = "2, 3"
+AbstractMCMC = "2, 3, 4"
 DensityInterface = "0.4"
 Setfield = "0.7.1, 0.8"
 julia = "1"