CompatHelper: bump compat for MemPool to 0.4, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the MemPool package from 0.2,0.3 to 0.2,0.3, 0.4. 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.