IMG4 File Format

There is an img4 file (sep-firmware.img4) found in /user/standalone/firmware on iOS 7.0.4 for the iPhone 5s. The IPSWFILE/Firmware/all_flash/all_flash.DEVICEIDENTIFIER.production for any A7 (64 bit) device and newer, 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