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

[github-actions]

This pull request changes the compat entry for the AbstractMCMC package from 3.2 to 3.2, 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.

[codecov]

Codecov Report

Merging #28 (a36e3cf) into main (47eac94) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main      #28   +/-   ##
=======================================
  Coverage   94.91%   94.91%           
=======================================
  Files           3        3           
  Lines          59       59           
=======================================
  Hits           56       56           
  Misses          3        3           

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