tvOS 11.4 can be downloaded over the air through the Settings app on the Apple TV by going to System --> Software Update. Apple TV owners who have automatic software updates turned on will be upgraded to tvOS 11.4 automatically.
The tvOS 11.4 update, paired with iOS 11.4, introduces support for AirPlay 2 functionality. AirPlay 2 is designed to bring multi-room audio to the Apple ecosystem, allowing the same music to be played on multiple devices throughout the home.
tvOS 11.4 turns the Apple TV into an AirPlay 2-compatible device that's listed in the Home app for HomeKit devices. AirPlay 2 playback can be controlled through the Apple TV, through an iOS device, or using Apple's personal assistant, Siri, to play content using voice commands.
AirPlay 2 is limited to the Apple TV and the HomePod at the current time, but it will expand to additional audio devices in the future.
tvOS 11.4 also introduces performance improvements, security enhancements, and bug fixes for issues that have been discovered since the release of tvOS 11.3.