Preview at WWDC likely in June, followed by September launch.
Apple Releases First OS X 10.11.4 El Capitan Beta to Public Beta Testers
The new beta is available through the Software Update mechanism in the Mac App Store for those who are enrolled in Apple's beta testing program. Those wishing to join the program can sign up on Apple's beta testing website.
OS X 10.11.4 has been provided to developers and public beta testers alongside OS X 10.11.3, which is also currently in testing. Apple has provided testers with two betas of OS X 10.11.3, and it could be released to the public shortly.
Both OS X 10.11.3 and OS X 10.11.4 appear to focus largely on under-the-hood bug fixes, security enhancements, and performance optimizations with few noticeable outward-facing changes. OS X 10.11.4 does support password protected notes in the Notes app, allowing a password to be assigned to individual notes.