Apple has posted four new developer betas: the fourth one of iOS 12.4, the third one of tvOS 12.4, the third one of watchOS 5.3, and the second one of macOS 10.14.6.
Registered developers can download the betas via Apple’s developer portal (https://developer.apple.com/account/#/welcome). Or they can be downloaded over-the-air once the proper configuration profile has been installed from the Developer Center.