This is the first release candidate of the new major version of Kryo, which fixes many issues and makes many long awaited improvements.
For details see this mailing list thread or check out the changes from 4.0.2 to 5.0.0-RC1.
So far there is only a minimal migration guide. We're asking the community to help and contribute this part: please edit the migration wiki page as you encounter any information or issues that might help others.
With the release candidate we're looking for feedback. If you think you found a bug, please submit an issue. If you think something should be changed before 5.0.0 is released, please post on the mailing list.
Compatibility
Due to big changes it is both source/binary incompatible and serialization incompatible.