Apple seeded the second beta of macOS High Sierra 10.13.4 to developers earlier this week, but it appears an updated version of the second beta was just released yesterday.
The original beta had a build number of 17E150f, while the new update available today from the Developer Center lists a build number of 17E150g. It’s not clear why Apple has released an updated version of beta 2, but the new beta is also available for public beta testers.
The update includes bug fixes and performance improvements for issues that weren’t addressed in macOS High Sierra 10.13.3, and it offers 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