iOS 11.4.1 beta 1 is now available to registered developers. If that's you, then you can download this update by going into Settings > General > Software Update on your device or by going to Apple's Developer Center on the web.
There's no word yet on what changes might be included in iOS 11.4.1. Considering that it's an x.x.1 release, though, we can likely expect bug fixes and under the hood improvements rather than any major new features.
Also released today is watchOS 4.3.2 beta 1. Like iOS 11.4.1, this update is likely focused on squashing bugs and improving performance.