Preview at WWDC likely in June, followed by September launch.
Apple Seeds First OS X 10.11.4 El Capitan Beta to Developers
The new OS X 10.11.4 beta can be downloaded through the Apple Developer Center.
It is not known what improvements the fourth update to OS X El Capitan will bring, but like prior updates, it's likely to focus on security enhancements, performance improvements, and bug fixes to address issues have been discovered since the release of OS X 10.11.2 and were not able to be included in the OS X 10.11.3 beta.
We'll update this post with any changes that are discovered in the beta.