Apple announced watchOS 2.0 for its new Apple Watch at WWDC 2015. Along with support for native apps and a new nightstand mode, the software update will bring a number of new features to the Apple Watch. With the public release set for sometime this fall, Apple continues to release beta updates to developers to allow them to get their apps ready for the launch.
watchOS SDK 2.0 beta provides support for developing watchOS apps. It is packaged with a complete set of Xcode tools, compilers, and frameworks for creating apps for watchOS. These tools include the Xcode IDE and the Instruments analysis tool, among many others.
This version of watchOS is intended for installation only on devices registered with the Apple Developer Program.
Apple this week seeded Apple watchOS 2.0 beta 4. The OTA update weighs in at 130MB. Among the changes are the reenabling of Apple Pay (after it was disabled with an earlier beta release), user interface improvements to the complications that use the Activity feature and a fix for an Apple ID bug with Messages. A number of known issues remain and will likely be addressed in a future beta release.
You must be registered developer to download watchOS 2.0 beta 4. You must also have iOS 9 installed on your iPhone.
Source : Apple Developer