Apple has released the RC versions of iOS 15.4, iPadOS 15.4, macOS 12.3, watchOS 8.5 and tvOS 15.4. All these are available to developers.
Apple Releases RC Builds of iOS 15.4, watchOS 8.5, tvOS 15.4 and macOS 12.3 to Registered Developers
We're getting very close to the full and final release of Apple's upcoming software and you can go ahead and download the RC (release candidate) build of everything right away if you're a registered developer with Apple.
In order to download the iOS 15.4 and iPadOS 15.4 RC updates right now, make sure you have a previous beta installed on your iPhone or iPad. Once that is done, go to Settings > General > Software Update and then tap on Download and Install. Even if you have iOS 15.3.1 and iPadOS 15.3.1 installed on your device, you can simply grab the iOS configuration profile from Apple's developer portal after which the RC update will be pushed to you over the air.
watchOS 8.5, tvOS 15.4 and macOS Monterey 12.3 RC builds are also available to download if you're a registered developer.
Are you enrolled in Apple's Beta Software Program as a public beta tester? These updates will be available to you as well in due time. For everyone else, it will be available in a week's time. If there are bugs found, Apple might even push out a second RC update before the final one. It's super rare, but it does happen.