Apple has released iOS 11.4 beta 1 for iPhone and iPad users enrolled in the iOS beta testing program.
The first iOS 11.4 beta release comes a few days after the final version of iOS 11.3 became available for general users.
Separately, Apple also released watchOS 4.3.1 beta 1, and tvOS 11.4 beta 1. Presumably a macOS High Sierra 10.13.5 beta 1 will be released shortly as well. The most recent final version of High Sierra is currently the freshly released macOS 10.13.4 build.
Users enrolled in the iOS beta testing program can find iOS 11.4 beta 1 through the Software Update mechanism of their devices. If you’re in the beta program but recently removed the iOS beta profile to stay on a stable build, you can get a new profile from the Apple Developer website for compatible iPhone or iPad models. Apple typically unveils a public beta release after the developer beta release.
As always, back up your iPhone or iPad before installing any system software, especially with beta releases.
iOS 11.4 betas are likely focusing on bug fixes and various enhancements, though a few new features are likely arriving as well. The iMessages in iCloud capability is apparently back again in iOS 11.4 after being pulled out of iOS 11.3 before the final release.
The most recent stable builds of system software remain iOS 11.3 for iPhone and iPad, tvOS 11.3 for Apple TV, watchOS 4.3 for Apple Watch, and macOS 10.13.4 for High Sierra Macs.