Beta 4 of iOS 12.4 & Beta 2 of MacOS 10.14.6 Available for Testing
Apple has released for testing iOS 12.4 beta 4, along with MacOS Mojave 10.14.6 beta 2.
These beta builds are entirely separate from the ongoing iOS 13 beta 1 and MacOS Catalina Beta 1, which are also in active beta testing for developers.
iOS 12.4 beta 4 likely focuses on bug fixes and minor enhancements for the iPhone, iPad, and iPod touch, and no major new features or changes are expected (there are plenty of new features in iOS 13 however, which will be released to the public this fall).
Anyone still currently enrolled in the iOS 12.4 beta can find the latest iOS 12.4 beta 4 release available now from the Software Update mechanism of the Settings app.
Likewise, macOS Mojave 10.14.6 beta 2 probably focuses mostly on bug fixes and minor enhancements for the Mac, and no major new features or changes are expected in that release either (but again, the upcoming MacOS Catalina release has many interesting new features and is also due this fall).
Any Mac users who are still currently enrolled in the MacOS Mojave beta testing program will find MacOS Mojave 10.14.6 beta 2 available now from the Software Update system preference.
Apple usually goes through several beta testing builds before issuing a final release to the general public.
Additionally, the most recent stable build of Mac system software is currently macOS Mojave 10.14.5.