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.
It's not yet clear what improvements the fourth official update to macOS High Sierra will bring, but it's likely to include bug fixes and performance improvements for issues that weren't addressed in macOS High Sierra 10.13.3. It may also include some of the features that are coming in iOS 11.3, such as support for Business Chat and unlimited ad-free access to music videos in Apple Music, which Apple says will be the new "home" for music videos.
According to Apple's release notes, macOS High Sierra 10.13.4 will begin sending warning notes when you open up a 32-bit app in preparation for phasing out 32-bit apps on the Mac. At this time, you'll see one warning letting you know that a Mac app is 32-bit, and it will popup when you open the app after installing the macOS High Sierra 10.13.4 update.
macOS High Sierra 10.13.4 also introduces the new smoke cloud wallpaper that was previously exclusively available on the iMac Pro.
The previous macOS High Sierra 10.13.3 update included security fixes and performance improvements, with no obvious outward-facing changes introduced.