Apple has stopped signing iOS 16.4 updates. This means that iPhone users who have already upgraded to the iOS 16.4.1 update cannot downgrade or revert to that firmware version.
Apple released iOS / iPadOS 16.4 update on March 28th, featuring 21 new emojis and optimized crash detection. The official changelog says this update also includes additional enhancements, bug fixes and security updates for the iPhone.
The main content of the iOS / iPadOS 16.4 update can be found in the related reading links below.