IMG4 Payload File
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What is an IM4P file?
An IM4P file is an IMG4 payload file used by iPad and iPhone mobile devices. It stores a payload for an IMG4 file, which is the actual content of the file such as the firmware key and iv.
The IMG4 format replaced the IMG3 file format. It is used on iOS devices running a 64-bit processor and the Apple Watch. The format is designed to contain IM4P files used by iOS' secure boot chain as well as signatures of the files. IMG4 files can be found in the /usr/standalone/firmware/ directory of the iOS file system.
NOTE: The IM4P file format was introduced with the release of iOS 7.0.1