Still not working for Watch series 6 on 11.3.1. Update checks don't find the 11.4 available.

Not just you. Switching devices in the prevue triggers the psychedelic display and complete system freeze.

This has happened before and is a bug in the beta 4 release tied to an expiration check for the valid timeframe for a build. In this case we are 9 days out from a weekly release cycle, someone at Apple screwed up and set the time check too soon. It’ll be fixed in the next beta release.