There are breaking changes in 4.0.0 and lots of improvements. One of the biggest is the policy schema is now managed externally and upstream. This decouples Phileas from the policy schema and allows both to develop independently and allows the policy schema to be used in other PII-focused applications.
See the release notes for more information: https://github.com/philterd/phileas/blob/4.0.0/RELEASE_NOTES.md
Full Changelog: 3.3.0...4.0.0