In my opinion the latest.
Make sure you read the full release notes of every intermediary release, as some contain upgrade steps (skipping those labelled ‘beta’ or ‘rc’)
When I say ‘read the full release notes’ please note, that the release list at https://github.com/mastodon/mastodon/releases only contains the beginning of each release note. You’ll need to click on the title of each release to read the full notes.
So: (1) read full release notes of all releases. (2) take a note of all steps required in each release. (3) check out 4.2.1. (4) execute all steps in order.
Don’t forget (0) make a backup though 😬