Designed for the fourth and fifth-generation Apple TV models, the new tvOS 11.4 developer beta can be downloaded onto the Apple TV via a profile that's installed using Xcode.
tvOS 11.4, paired with iOS 11.4, reintroduces AirPlay 2 features that were present in early tvOS and iOS 11.3 betas but were removed ahead of release. With AirPlay 2, the same audio content can be played on multiple devices (like the Apple TV) throughout your home, and audio playback can be controlled via the iPhone or through Siri commands. After installing iOS 11.4 and tvOS 11.4, the Apple TV is also once again be listed in the Home app.
There were no other new features discovered in the first two tvOS 11.4 updates, and it's likely it focuses on bug fixes and other small improvements. Apple's tvOS updates have historically been minor in scale, and we may not find any other new additions.
We'll update this post should new features be introduced in the fourth beta of tvOS 11.4.
Update: Apple has released tvOS 11.4 beta 4 for public beta testers.