Preview at WWDC likely in June, followed by September launch.
Apple Seeds First watchOS 4.1 Beta to Developers With Apple Music Streaming, New Radio App
Once the proper configuration profile has been installed from the Apple Developer Center, the new watchOS 4 beta can be downloaded through the dedicated Apple Watch app on the iPhone by going to General --> Software update.
To install the update, the Apple Watch needs to have at least 50 percent battery, it must be placed on the charger, and it has to be in range of the iPhone.
watchOS 4.1 introduces support for streaming music from Apple Music or iCloud Music Library straight to the Apple Watch. There's also a new Radio app that offers up access to Beats 1 live or any Apple Music radio station on the new cellular Apple Watch Series 3 models.