Beta testers who have signed up for Apple's beta testing program will be able to download the new macOS High Sierra beta through the Software Update mechanism in the Mac App Store.
Those who want to be a part of Apple's beta testing program can sign up to participate through the beta testing website, which gives users access to iOS, macOS, and tvOS betas.
macOS High Sierra 10.13.4 introduces support for some features that are also 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.
The new macOS update also includes the smoke cloud wallpaper that was previously only available on the iMac Pro, it changes the name of the "iBooks" app to just "Books," 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.