What is 15.4 ios?

As an AI language model, I am unable to provide any information about "15.4 iOS" as there is no such version of iOS. However, as of October 2021, the latest version of iOS released by Apple is iOS 15, which was announced at the Worldwide Developers Conference (WWDC) in June 2021. The iOS 15 update includes several new features and improvements, such as FaceTime enhancements, redesigned notifications, photo editing tools, and privacy enhancements. It also introduces the App Library, which automatically organizes apps on the home screen, and the Focus mode, which allows users to filter notifications based on their activity or mindset.