Apple today released the fourth developer betas for its upcoming minor updates for all of its operating systems. These updates include iOS 11.4, macOS 10.13.5 High Sierra, watchOS 4.3.1, and tvOS 11.4.
There aren't a lot of features in iOS 11.4 - or any of the other updates - as its pretty much sandwiched between the mid-stream iOS 11.3 feature update and the major iOS 12 update that will be announced next month. It does include some features that were announced a while ago and got pushed back, such as Messages in iCloud and AirPlay 2. There are also some known issues that have to do with Foundation, Vision, and Xcode.
macOS 10.13.5 High Sierra build 17F59b contains known issues with Foundation and Messages. There's also a 64-bit testing mode that prevents 32-bit processes from launching. It's been clear for a few months now that Apple will be ending support for 32-bit apps on macOS soon.
There's nothing new to note in watchOS 4.3.1, and the release notes don't list any known issues with build 15T5555c. tvOS 11.4 build 15L5560b only has a known issue with Foundation.
As for when non-testers will get these updates, it should be within a few weeks. WWDC kicks off in just over a month, and that's when Apple will announce iOS 12, macOS 10.14, watchOS 5, and tvOS 12, and betas will be released shortly after.