The macOS Mojave update can be downloaded using the Software Update mechanism in the Mac App Store after downloading the appropriate profile from Apple’s beta testing website. Apple’s beta testing site gives users access to iOS, macOS, and tvOS betas.
The update also includes support for Touch ID-enabled Safari AutoFill and automatic Dark Mode themes in Safari. That means if you have Dark Mode enabled on macOS Mojave, when you visit a website with a dark theme, it will be activated automatically. You can see a demo of the feature here.
macOS Mojave 10.14.4 will likely be in beta testing for the next several weeks as Apple refines features and works out bugs. After that, it will see a release alongside iOS 12.2, watchOS 5.2, and tvOS 12.2.