Apple has stopped signing iOS 15.4 just over a week after releasing iOS 15.4.1 to the public. This means it is no longer possible to downgrade your iPhone to the older version of the operating system. The company also stopped signing iPadOS 15.4, so you cannot downgrade your iPad to the older firmware either.
Apple no longer allows you to downgrade your iPhone to iOS 15.4

Photo: Ed Hardy/Cult of Mac