The new macOS Mojave 10.14.4 beta can be downloaded through the Software Update mechanism in System Preferences after the proper profile has been installed from Apple's Developer Center.
macOS Mojave 10.14.4 brings Apple News to Canada for the first time, allowing Canadian Mac users to access news stories in French, English, or both.
The update also includes support for Safari AutoFill using Touch ID and automatic dark mode themes in Safari. That means if you have Dark Mode enabled, when you visit a website that has an option for 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.
Update: Apple has also released a new 10.14.4 public beta for its public beta testers.