There is only one file type that uses the .MI4 extension:
Audio Player Firmware File
Firmware file used by various audio players that have the PortalPlayer chipset, including SanDisk, Philips, Samsung, and older iPod players; contains low-level software functions and themes that are used to run the audio player.
MI4 files are used to upgrade or change the themes of an audio player.
Updated: August 14, 2010