The fourth OS X 10.11.6 beta update can be downloaded through the Apple Developer Center or the Software Update mechanism in the Mac App Store.
OS X 10.11.6, much like OS X 10.11.5 and the El Capitan updates that came before it, focuses mainly on performance improvements and bug fixes instead of obvious outward-facing changes. According to Apple's release notes for earlier betas, the update improves the stability, compatibility, and security of Macs.
No obvious changes or bug fixes were found in the first three betas, but we'll update this post should anything new be discovered in the fourth beta.
Apple has also started testing macOS Sierra, the next-generation Mac operating system set to be released this fall. macOS Sierra includes features like Siri integration, auto unlocking with the Apple Watch, Apple Pay for the web, a new storage optimization feature, and more.