The different HUMAnN 3.x releases are each compatible with a separate MetaPhlAn 4 database (please see their release notes for details). However, we have not released a HUMAnN 3.x yet for the latest (Jan25) MetaPhlAn 4 database, and I’m not sure if we will given that we are shifting focus to HUMAnN 4 at this point.
You and others here are welcome to reply and let us know if maintaining backward compatibility is important for your workflows. If there’s a lot of interest that would provide more motivation for another HUMAnN 3.x release.