Preview at WWDC likely in June, followed by September launch.
Apple Seeds Sixth Beta of macOS Sierra 10.12.4 to Developers [Update: Public Beta Available Too]
The sixth macOS Sierra 10.12.4 beta is available for download through the Apple Developer Center or the software update mechanism in the Mac App Store for those who have previously installed a beta.
macOS Sierra 10.12.4 brings iOS's Night Shift mode to the Mac for the first time. First introduced on iOS devices with iOS 9.3, Night Shift is designed to gradually shift the display of a device from blue to yellow, cutting down on exposure to blue light. Blue light is said to disrupt the circadian rhythm and is believed to interrupt sleeping patterns.
Night Shift can be activated through the Displays section of System Preferences, where a setting to have it come on at sunset and turn off at sunrise is available. Night Shift can also be toggled on manually through the Notification Center or via Siri.
The 10.12.4 update focuses mainly on Night Shift, but also includes dictation support for Shanghainese, cricket scores for Siri, improved PDFKit APIs, and iCloud Analytics options.
Update: Apple has also made the sixth beta of macOS Sierra 10.12.4 available for public beta testers.