Preview at WWDC likely in June, followed by September launch.
Apple Seeds Fourth OS X 10.11.5 El Capitan Beta to Developers and Public Beta Testers
The fourth beta of OS X 10.11.5 can be downloaded through the software update mechanism in the Mac App Store or through the Apple Developer Center (developers only).
OS X 10.11.5 focuses on security enhancements, performance improvements, and bug fixes to address problems that have surfaced since the release of OS X 10.11.4. As with prior betas, the changes appear to be minor in scope, and no obvious outward-facing tweaks have been discovered in the first three OS X 10.11.5 betas.
We'll update this post if any new features are found in the fourth beta.