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

[github-actions[bot]]

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.

[coveralls]

Pull Request Test Coverage Report for Build 2698755089

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 69.943%

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Totals
Change from base Build 2698211386:	0.0%
Covered Lines:	1950
Relevant Lines:	2788

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💛 - Coveralls