Apple has released macOS Sierra 10.12.6
beta 4 with a build number 16G18 to developers for testing.

The macOS Sierra 10.12.6 update improves the stability, compatibility, and
security of your Mac.

macOS Sierra 10.12.6 beta 4 is now
available for download to registered developers via Mac App Store’s Updates tab.
Apple has released macOS Sierra 10.12.6 beta 4 with a build number 16G18 to developers for testing.
It is also available Apple Developer Program on
Apple’s website.

Final release of macOS Sierra 10.12.6
will be available for Public very soon later this month and will get the
notification on their mac.