The current update to New High Sierra for Mac appeared the next day after the release of the previous version. The 10.13.3 did not introduce any outward-facing major improvements; instead, it focused on security issues and featured a few performance boosters. In particular, the fixes are intended to combat the revealed bugs related to the Spectre and Meltdown vulnerabilities, and the bug with the App Store password.
What advantages should we expect from this recent update? It is not clear yet what novelties the fourth official update will offer the users. But we may expect that part of them will be bug fixes and various performance goodies as was the case with the 10.13.3 version.
Also, the 10.13.4 may introduce (similar to the iOS 11.3) such a new feature as support for Business Chat. Another possible advantage may become the unlimited ad-free access to music videos in Apple Music. Another novelty also brings the new update closer to the iMac Pro. It is the smoke cloud wallpaper that was first introduced and has been exclusively available on that computer.
In the light of the current phase-out of the 32-bit apps, warning messages will be sent in the form of pop-ups to the users to inform that the Mac app they are trying to install is 32-bit. And, according to the release notes, it is the 10.13.4 update of the macOS High Sierra that will start sending such notes. The phase-out began in January 2018, and, by June 2018, both apps and their updates submitted to Mac App Store must be 64-bit.
As usual, developers with registered profiles are welcome to download the beta either via Apple Developer Center or via Software Update system.