It's only been a few days since Apple released iOS 11.3, watchOS 4.3, and tvOS 11.3, but it's already time for new betas of the next minor updates. iOS 11.4, watchOS 4.3.1, and tvOS 11.4 are available for developers to test.
The release notes don't say anything about what's new in terms of features, and that's not exactly surprising. The last round of updates were mid-stream feature updates, and the next ones that will be announced will be the major feature updates at WWDC, such as iOS 12. It's safe to assume that there's not a lot new today, except for the likely addition of Messages in iCloud, which Apple has been promising since it announced iOS 11 last year, and has only made appearances in beta software.
In iOS 11.4 build 15F5037c, there are of course known issues. When using AirPlay to stream a video from YouTube, the audio might lag behind the video feed by 2-3 seconds, or the video might not play at all. Playback might get stuck at the end of a track in Apple Music as well, and the rest of the issues are mostly developer-related.
As far as watchOS 4.3.1 build 15T5535b goes, there's nothing of value in the release notes. This also isn't particularly surprising, given the version number's indication that this is just a stability update.
Finally, tvOS 11.4 build 15L5536c has some known issues that have to do with Mobile Device Management and Foundation.
Those on the public betas should receive these updates within a couple days. If you don't subscribe to beta software, you can probably expect to see them next month.