Preview at WWDC likely in June, followed by September launch.
Apple Seeds Second OS X 10.11.4 El Capitan Beta to Developers
The second OS X 10.11.4 beta can be downloaded through the Apple Developer Center or via the Software Update Mechanism in the Mac App Store.
OS X 10.11.4 includes a couple of new features, such as the ability to support password protected notes in the Notes app, but like the recent OS X 10.11.3 update, it appears to focus primarily on under-the-hood bug fixes and performance improvements with few noticeable outward-facing changes.
We'll update this post with any new features that are discovered in the second beta of OS X 10.11.4.