How to map streams to specific output streamIDs
It's using a counter! To get it to be stream #5, map 5 streams before it :)

That said, mp4box will allow you to set a custom track ID.

mp4box -set-track-id 1:5 file.mp4


This changes existing track with ID 1 to 5. Use mp4box -info file.mp4 to get existing IDs.

However, this will still show up as stream 0 in ffmpeg. But mediainfo and mp4box will show it as having ID 5.