This is a pre-release version of OS X v10.11.4. Please refer to the release notes for complete installation instructions.
Apple has told testers to focus on iBooks, Messages, Notes, and Photos. 10.11.4 brings secure notes, live photos to messages, and other stability/performance improvements.
The company also released OS X Server v5.1 beta 6 today.
OS X Server 5.1 beta 6 (build 15S5127, app version 5.1) is now available for testing. This version requires OS X El Capitan 10.11.4 beta 7 (build 15E64a).
You can download the OS X Software Update Seed Configuration Utility from the link below.