Following the March 31 release of iOS 15.4.1, Apple has stopped signing iOS 15.4, the previously available version of iOS that came out earlier in March. As iOS 15.4 is no longer being signed, it is not possible to downgrade to that version of iOS once you've installed iOS 15.4.1.

Apple routinely stops signing older versions of software updates after new releases come out in order to encourage customers to keep their operating systems up to date, so it is not unusual that iOS 15.4 is no longer being signed.

iOS 15.4 was a major update featuring support for unlocking an iPhone while wearing a mask, new emoji characters, Universal Control on the iPad, and tons more.

iOS 15.4.1 added a fix for a battery drain issue on the ‌iPhone‌, but more importantly, it addressed AppleAVD issue that could allow an application to execute arbitrary code with kernel privileges. Apple said this exploit may have been actively exploited, so it is a good idea to update to iOS 15.4.1 if you have not done so already.

Apple has also stopped signing tvOS 15.4 and HomePod 15.4 software.

Source: Macrumors