The new macOS High Sierra 10.13.4 beta can be downloaded from the Apple Developer Center or through the Software Update mechanism in the Mac App Store with the proper profile installed.
macOS High Sierra 10.13.4 includes bug fixes and performance improvements for issues that weren't addressed in macOS High Sierra 10.13.3.
The update offers support for some features that are available in iOS 11.3, like Messages on iCloud, which uploads all of your iMessages to the cloud. It will also support Business Chat, a feature coming when iOS 11.3 and macOS 10.13.4 are released to the public, and it includes improved support for eGPUs.
The macOS 10.13.4 update also brings the smoke cloud wallpaper that was previously only available on the iMac Pro and it introduces a warning when opening up a 32-bit app as part of an effort to phase them out.
In the future, Apple plans to phase out 32-bit Mac apps, just like it did with 32-bit iOS apps. Apple says macOS High Sierra is the last version of macOS that will support 32-bit apps without compromises.
Update: Apple has also made a new beta of macOS High Sierra 10.13.4 available to its public beta testers.