Apple has released the fourth developer betas of macOS 10.13.4 and iOS 12.1.3.
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.
Apple has also released the fourth public beta of iOS 12.1.3. The public betas are available on Apple’s Beta Software Program website.