"Hey Siri" support and possibly wireless charging case alongside AirPower charging mat.
Apple Releases Fourth OS X 10.11.4 El Capitan Beta to Public Beta Testers
The fourth 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 focuses primarily on under-the-hood bug fixes, security enhancements, and performance optimizations, but there have been a few outward-facing changes. In the Notes app, OS X 10.11.4 offers both password protected notes and an import function for importing Evernote notes. The OS X 10.11.4 update also includes Live Photos support in the Messages app.
Apple is likely to release OS X 10.11.4 in the spring, alongside iOS 9.3, watchOS 2.2, and tvOS 9.2.