The fourth beta of macOS Sierra 10.12.6 can be downloaded through the Apple Developer Center or through the Software Update mechanism in the Mac App Store.
We didn’t find any significant features or notable bug fixes in the first three macOS Sierra betas, and because Apple does not provide beta release notes, it’s likely we won’t know what’s included in the update until it sees a public release.
macOS Sierra 10.12.6 is likely to be one of the final updates to the Sierra operating system as Apple transitions to macOS High Sierra, which was introduced at the Worldwide Developers Conference in June.
Update: A fourth public beta of macOS Sierra 10.12.6 is also available for public beta testers.