This being the 4th beta indicates that the official release might be around the corner, and also because this release was just one week after the previous one.
In this build, Apple has ironed out bugs and has polished the following features:
- Graphic Drivers
- FaceTime Audio
If you are a registered developer, download the beta here: https://developer.apple.com/osx/whats-new/
The official release could be out within a week.