Apple seeded the tvOS 11.3 developer beta 6 on Tuesday. This update is believed to include support for Dolby Vision on some Sony TVs, and it may include AirPlay 2, as well. That feature surfaced in early developer betas of tvOS 11.3, but was pulled. It also includes UI enhancements for Match Content and other areas.
tvOS is designed for Apple TV 4th and 5th generation devices, including Apple TV 4K. Installing developer betas of tvOS is a more complicated process than with other Apple devices. The company has a detailed how-to using both Xcode and Apple Configurator on your Mac.