The new OS X 10.11.4 beta, being just an incremental 10.11.x release, probably isn’t going to include a ton of new features, but more than likely will have some bug fixes and improvements with security and performance baked into the operating system.
It’s worth noting that Apple’s OS X 10.11.3 beta is still in beta and hasn’t yet been released to the public. Much like OS X 10.11.4, OS X 10.11.3 only features under-the-hood improvements that aren’t documented in detail.
Developers can grab the update from the developer center on Apple’s web site. After installing, all future updates can be grabbed from the “Updates” tab in the Mac App Store as they’re released over the course of the next several weeks.
We will update this post as new features are uncovered, so stay tuned!