In addition to releasing iOS 9 beta 4 and OS X 10.11 El Capitan beta 4 to its developer army, a matching beta of the upcoming Apple Watch software update is now available to registered members of the Apple Developer Program.
The new watchOS 2 beta 4 (build 13S5305d) requires an iPhone running iOS 9 beta 4 and can be applied over-the-air by going to the General -> Software Update section of the Apple Watch companion app.