Previewed at WWDC, launching in the fall.
Apple Releases First Beta of iOS 11.4 for Public Beta Testers
Beta testers who are members of Apple's beta testing program will receive the iOS 11.3 beta update over-the-air after installing the proper certificate on an iOS device.
Those who want to join the beta testing program can sign up on Apple's beta testing website, which gives users access to iOS, macOS, and tvOS betas. iOS betas are not always stable and should not be installed on a primary device.
iOS 11.4 is going to look a lot like iOS 11.3 because several features that were removed from iOS 11.3 ahead of its release have been re-introduced in iOS 11.4.
The update includes support for AirPlay 2 features, allowing you to play the same song on multiple devices and adding the Apple TV to the Home app, plus it offers Messages on iCloud.
Messages on iCloud was present throughout the iOS 11.3 beta testing period, but it did not make it into release. The feature is designed to let you store your iMessages in iCloud, syncing them across devices and saving valuable storage space.
Stereo sound in HomePod is also an available feature, but it is not functional at this time as Apple has not provided the 11.4 HomePod software to developers or public beta testers.