MacRumors published a story that the first beta version of watchOS 10.4 is now available for registered developers.
Apple Seeds First Beta of watchOS 10.4 to Developers
Apple today seeded the first beta of an upcoming watchOS 10.4 update to developers for testing purposes, with the beta coming a week after the release of watchOS 10.3.
To install the watchOS 10.4 update, developers need to open the Apple Watch app, go to the Software Update section under "General" in Settings, and toggle on the watchOS 10 Developer Beta. An Apple ID linked to a developer account is required.
