The new macOS Catalina beta can be downloaded using the Software Update mechanism in System Preferences after installing the proper software from the Developer Center.
macOS Catalina 10.15.4 includes Screen Time Communication Limits, a feature first brought to the iPhone in the iOS 13.3 update, and it brings a new Head Pointer Accessibility option that allows the cursor to be controlled with head movements using the Mac's camera.
Real-time lyrics are included in macOS 10.15.4, bringing an iOS feature introduced in iOS 13 to the Mac. Real-time lyrics allow the lyrics for a song to scroll in real-time to the music so you can follow along with what's being sung.
References to AMD processors have also been discovered in the macOS 10.15.4 beta, leading to speculation that Apple is working on AMD-based Macs, but these references could simply be for internal testing rather than evidence of an AMD Mac.
Though not directly related to macOS Catalina 10.15.4, Apple is adding a new universal purchase option for macOS and iOS apps, which will allow Apple device users to purchase one app that works across multiple platforms.