The IMG4 file format is a replacement for the IMG3 file format and is used on devices running a 64-bit processor (A7 or newer). It's also used across the entire Apple Watch series, in both the armv7k and arm64_32 processors. Just like the old IMG3 file format, it is used to contain files used by iOS' secure boot chain (im4p files), signatures of those files (im4m/
ApImg4Ticket files) and some info used in the Restore Process ("
IM4R" can be found in MobileDevice framework). A complete IMG4 file is the result of concatenating the payload, manifest and restore info (aka boot nonce) in one file and wrapping it with an IMG4 header.
Contrary to the IMG3 file format where a custom binary format was used, IMG4 files (and their payloads) are DER encoded ASN.1 objects. The reason for Apple's choice of ASN.1/DER encoding over their own binary Property List (PList) file format is unknown.
sequence [ 0: string "IMG4" 1: payload - IMG4 Payload, IM4P 2:  (constructed) [ manifest - IMG4 Manifest, IM4M ] ]
Payload contains the actual content of the file. IMG4 format allows to store payload in a separate file with im4p extension. im4p files are used more often than complete IMG4 files. Payload is structured as follows:
sequence [ 0: string "IM4P" 1: string type - ibot, rdsk, sepi, ... 2: string description - 'iBoot-1940.1.75' 3: octetstring - the encrypted/raw data 4: octetstring - containing DER encoded KBAG values (optional) sequence [ sequence [ 0: int: 01 1: octetstring: iv 2: octetstring: key ] sequence [ 0: int: 02 1: octetstring: iv 2: octetstring: key ] ] ]
Item 0 in the KBAG sequences is 01 for the production KBAG and 02 for the development KBAG. Development KBAGs can only be decrypted with "development" devices (Apple-internal prototypes or demoted production devices), since they use a different GID. The result of that decryption is the same iv+key for either dev or prod KBAGs.
A.k.a. IM4M. Contains signature for one or multiple IMG4 payloads. Manifests are designed in a way that they can be stored separately from an IMG4 file or be a part of one. An example of an IMG4 manifest is ApImg4Ticket. Each IMG4 manifest is made of tags structured as follows (content = DER encoded object):
[type] (private) [ - DER tag's class is the type of the tag (4 bytes) encoded as a big-endian integer sequence [ string type - type of the tag, 4 bytes content - content of the tag ] ]
Manifest structure (tag <type> = tag of type 'type'):
sequence [ 0: string "IM4M" 1: integer version - currently 0 2: set [ tag MANB [ - manifest body set [ tag MANP [ - manifest properties set [ tag <manifest property> [ content ] ... - tags, describing other properties ] ] tag <type> [ - ibot, illb, sepi, krnl, NvMR, bbcl... set [ tag <tag property> [ content ] ... ] ] ... - tags for other images ] ] ] 3: octet string signature 4: sequence [ - containing certificate chain (arbitrary number of certificates) certificates ]