What breaking changes were introduced in 4.0.0?

[migg24]

I cannot find a changelog, version 4 has not even a tag or a release, the PR for version 4 has no info.

Where can we find info about what changed in version 4 so we can update?
Please add some documentation about this.

[marc1404]

While I also support having a dedicated CHANGELOG to make things like breaking changes visible at least I could see that the commits of the v4 release have semantic labels. See the "BREAKING" commit here:
b8d4692 (#21)