Apple has released the beta 4 of macOS Sierra 10.12.6 for paid betters. In macOS Sierra 10.12.6, bug fixes are included, but new features are not included. This operating system version should be the last one before macOS High Sierra.
The Betas appear almost every week and so it is not surprising that Apple now offers the beta 4 of macOS Sierra 10.12.6 for download. First, the version is always for paid developers, while a few days later the public testers are allowed to try the operating systems.
There are no new features of macOS Sierra 10.12.6 available yet. It should be the last version before macOS High Sierra, which is expected for the autumn.
Apple had announced on the occasion of the WWDC 2017 iOS 11, macOS 10.13 as well as tvOS 11 and watchOS 4.0 and a look at the new functions granted. The beta program for paid developers started right afterwards. Still, there are no public betas from the whole new operating systems. They are expected at the end of June 2017.