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.

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

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