The Fourth Beta is available through the Software Update mechanism in the Mac AppStore 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 **tes app, OS X 10.11.4 offers both password protected **tes and an import function for importing Ever**te **tes. 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.