There are no breaking changes in this release. If you are already using version 4.x, you can use version 5 immediately with zero code changes. 🎉 Version 5 will pick up where the 4.x roadmap (now the 5.x roadmap) left off.
The only reason for the major version bump has to do with how we were specifying dependencies in react-router-dom
As in: https://reacttraining.com/blog/react-router-v5/
This is obviously not urgent, just let's remember that 5 is out, it's fully backwards compatible and solves this known issue that might bite us or our users.
Let's just remember to fix this (easily fixable) deprecation before upgrading: https://github.com/zambezi/caballo-vivo/issues/23.