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.
Convert BMW iDrive multimedia data to MPEG-4 video file format.
We found 3 software records in the database related to .br4 to .mp4 file format conversion.
Microsoft Windows software - convert br4 to mp4 on Windows
A command line converter for BMW multimedia files
Convert from br4 file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned br4 to mp4. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A BRX to audio converter
Apple macOS / Mac OS X software - convert br4 to mp4 on OS X
An audio data converter for Mac used to convert audio from BMW car system to MP3, WMA and MP4