Apple has pushed the update three weeks after their third update to macOS Catalina. While Apple has released the fourth developer beta of macOS Catalina, the fourth developer betas of iOS 13 and watchOS 6 are nowhere to be found. They might still come later today, or Apple could be waiting a bit longer.
Apple has a warning for the users who are willing to update,
Using Xcode 11 beta 3 with macOS Catalina beta 4 will prevent SwiftUI previews from working. Update to Xcode 11 beta 4 when it becomes available.
The build can be downloaded by via the Apple Developer Center, or as an over-the-air update.
As with all new betas, macOS Catalina should not be installed on a primary machine because it is early release software and has bugs.