Preview at WWDC likely in June, followed by September launch.
Apple Seeds Fifth OS X 10.11.4 El Capitan Beta to Developers and Public Beta Testers
The fifth OS X 10.11.4 beta, build 15E56a, can be downloaded through the Apple Developer Center (developers only) or via the Software Update Mechanism in the Mac App Store (for developers and public beta testers).
OS X 10.11.4 offers a couple of new features, such as Live Photos support in Messages, the ability to password protect notes in the Notes app, and an option to import notes from Evernote, but like the recent OS X 10.11.3 update, it appears to focus primarily on under-the-hood bug fixes and performance improvements. Almost all of Apple's OS X updates to date have been smaller updates that improve performance rather than introduce new features.
We'll update this post with any features or significant changes that are discovered in the fifth beta of OS X 10.11.4. OS X 10.11.4 is likely to see a spring release alongside iOS 9.3, tvOS 9.2, and watchOS 2.2, with the new software coming as early as March 21, the date of Apple's planned spring event.