CompatHelper: bump compat for FastDifferentiation in [weakdeps] to 0.4, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the FastDifferentiation package from 0.3.15 to 0.3.15, 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.

[moble]

As noted in #66, this will have to wait on an improvement to FastDifferentiation, and will probably require a further bump to 0.4.2 or something as the minimum in the 0.4 branch.