You can use the free bmwconv command line tool to convert BR4 BMW iDrive multimedia data files to common MPEG-4 format (.mp4). There doesn't seem to be any other software that could help you to convert BR4 to MP4.
Open BR4 file Open MP4 fileA command line converter for BMW multimedia files
A BRX to audio converter
An audio data converter for Mac used to convert audio from BMW car system to MP3, WMA and MP4