Apple recently released iOS 15.4.1, and as is traditional, Apple has now stopped signing iOS 15.4, making downgrading to that version of iOS no longer possible.
Apple earlier this month released iOS 15.4,1. iOS 15.4.1 fixes an issue that could cause the battery to drain more quickly than expected after updating to iOS 15.4. The update also includes fixes for Braille devices and Made for iPhone hearing devices.
The new software can be downloaded on compatible devices over the air by going to “Settings” -> “General” -> “Software Update.”
Falling back to an older iOS build is often used by users that jailbreak their iPhones. Restoring an iPhone or iPad to a previous version of iOS can also sometimes be helpful for users who experience significant bugs after upgrading to the latest version of iOS.