Only three days after releasing the third beta versions of iOS 12.1.3 It's from macOS Mojave 10.14.3, Apple today released the fourth test versions of these systems (builds 16D5039a and 18D39a respectively). For now, the new versions of watchOS 5.1.3 and tvOS 12.1.2, which continue in beta 3, are still missing.
For now, nothing very relevant has been discovered in these updates and the version numbers themselves even indicate that these are updates to make bug fixes and general enhancements. Precisely for this reason, we can expect all of them to be released even further in January if we continue at this pace of release.
The two updates have also painted on the Apple Beta Software Program so that all registered users and not just developers can test and send the appropriate Apple feedbacks.