[Libav-user] ffmpeg ; splitting an input movie into multiple tracks in a single output movie, possible?

["René J.V. Bertin"]

I discovered yesterday that it's possible to split an input movie (a raw m4v stream) into 4 separate output movies (.mov) each containing a quarter of the original image (obtained with -vf crop=w/2:h/2:x:y), using a single invocation of ffmpeg.
Is it possible to generate a single output movie with 4 tracks instead, either in that same invocation (would make most sense, I presume) or else by piping the output into a separate ffmpeg command?

Thanks in advance!
René