IMG4 File Format

There is an img4 file (sep-firmware.img4) found in /usr/standalone/firmware on iOS 7.0.1 for the iPhone 5s. The IPSWFILE/Firmware/all_flash/all_flash.DEVICEIDENTIFIER.production for any A7 (64 bit) device and newer as well as Apple Watch, contains an IM4P version of the same file. The file is an DER encoded ASN.1 object and structured as follows:

Header
30 - type tag indicating SEQUENCE 8 - ? 3  - 2,3,4 length in octets of value that follows (length of next value; size of file) x octets - size of file from here 16 - indicating an IA5String to follow 04 - length in octets of IA5String that follows 49 4D 47 34 - IA5String, the magic "IMG4" ... - sequential data