Apple has seeded watchOS 9 beta 4 to developers for testing. The build number is 20R5332g.
WatchOS 9 features more customizable watch faces, an enhanced Workout app, sleep stages, an AFib History feature, and an all-new Medications app, and more. Check what's new in watchOS 9 here.
To update to the latest beta version of watchOS, install the configuration profile on Apple Watch and use the Watch app on iPhone to update the software.