The sixth OS X 10.11.4 beta, build 15E61b, can be downloaded through the Apple Developer Center or via the Software Update Mechanism in the Mac App Store.
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 sixth 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.