Marshmallow v4 is out!

Compared to marshmallow 3, it contains less breaking changes, so the upgrade should be smooth to most users.

The removal of deprecated features makes the code easier to maintain and unlocks future enhancements.

See the changelog for a full list of new features and removals: https://marshmallow.readthedocs.io/en/latest/changelog.html

And the upgrading guide for a comprehensive list of changes to apply to user code: https://marshmallow.readthedocs.io/en/latest/upgrading.html

Marshmallow v4 is already supported by most libraries in the ecosystem.

Thanks to all contributors for making this possible.