Here are Apple’s iOS 18.4 release notes

iOS 18.4
Apple has released the second release candidate (RC) for iOS 18.4 and iPadOS 18.4 to developers, following the first RC by a few days. These updates, available via Settings > General > Software Update on compatible devices, introduce several features.
Notably, iOS 18.4 includes Priority Notifications, an Apple Intelligence feature that highlights important notifications on the Lock Screen.
Additionally, Apple News+ subscribers gain a new Food section, Image Playground adds a Sketc