It's been exactly two weeks since Apple released the first developer betas for iOS 11.4, watchOS 4.3.1, and tvOS 11.4, which is that it's time for the second developer betas. There aren't a lot of new front-facing features in these updates. They're really just a sort of wrap-up before the big updates like iOS 12 are announced in June.
Some of the key features that you'll find in iOS 11.4 are Messages in iCloud and AirPlay 2, which were both announced last year. They kept getting delayed, and Messages in iCloud showed up in betas for both iOS 11 and iOS 11.3, but never made it into the final product. It would seem that this round of updates is meant to wrap up all of Apple's promises for iOS 11 and its counterparts.
Today's iOS build, 15F5049c, resolves issues that have to do with AirPlay, Keychain, and Music. There are still known issues with Foundation, Vision, and Xcode, but none of them are new to this build.
watchOS 4.3.1 build 15T5545b doesn't have anything listed in the release notes, whether it has to do with new features or fixes and issues. tvOS 11.4 build 15L5546a resolves issues with Mobile Device Management, and still has a known issue with Foundation.
If you're on the iOS or tvOS public betas, you'll probably see these builds either today or tomorrow. If you're not on the beta programs, you'll probably get these updates next month, shortly before WWDC.