Apple has today released macOS Catalina 10.15.6 Beta 4 to registered developers ten days after the release of macOS Catalina 10.15.6 Beta 3 and over a month after the public release of macOS Catalina 10.15.5 and macOS High Sierra 10.13.6.
The build number for this update is 19G71a. The letter ‘a’ at the end of the build number indicates that this is the last beta version for macOS Catalina 10.15.6. macOS Catalina 10.15.6 will be one of the last few updates to macOS Catalina, as Apple will shift its primary focus to macOS Big Sur. macOS Catalina 10.15.6 is expected to be released to the public by next week. According to Apple’s release notes, there are no new changes in macOS Catalina 10.15.6 Beta 4.
You need to be registered for the macOS Catalina Developer Beta program to update your Mac to this beta through System Preferences.