When the iOS 15.4 update was released, some users complained of battery drain issues that suddenly plagued older and newer iPhones.
Everyone is recommended to update to iOS 15.4.1 immediately because it also fixes an AppleAVD issue that could allow apps to execute arbitrary code with kernel privileges. It is an exploit that Apple said was commonly used to breach affected devices.
Considering what it brings, the update file size is small, around 320MB thereabouts. To update, go to Settings> General> Software Update.
iOS 15.4 brought with it Face ID while wearing a mask, which is a much-needed feature in this post-pandemic world where face masks are a norm. Take note that this feature is only available on iPhone 12 or later devices.