Apple has stopped installing iOS 15.4. Users will not be able to Downgrade, Signing, or Restore back to that iOS version.
Disable downgrade back to iOS 15.4
Apple has disabled the installation of the following versions of iOS, iPadOS:
That means the user Unable to Downgrade, Signing, Restore, or Install Firmware iOS 15.4 (iPadOS 15.4) At this time, it can only install iOS 15.4.1 (8 Apr 65)
As for iOS 15.4 users, it can still be used normally. But if updated to iOS 15.4.1, then it will not be able to downgrade back to iOS 15.4.
OS 15.4 comes with features such as scanning your face (Face ID) while wearing a mask. But in this version, many users encounter problems with the battery draining faster than usual, causing Apple to release iOS 15.4.1 to fix the battery problem.
Therefore, it is recommended that users update iOS to version 15.4.1 or the latest version is best.
Summary of new features iOS 15.4.1 : https://www.iphonemod.net/ios-15-4-1-released.html