Apple has released the fourth developer beta version of watchOS 2 alongside the latest beta of iOS 9. As a reminder, you'll need to have the iOS 9 beta installed on your device in order to install the watchOS beta.
watchOS 2 is expected to arrive later this year, and it will bring with it a number of improvements for the Apple Watch. Developers will be able to write apps that run natively on the watch, and take advantage of hardware features like the digital crown and heart rate monitor. Developers will also be able to build their own complications for watch faces that support that feature.