But it isn't either 4:3. If I would crop it to this ratio, too much picture content would be missing.It's not 16:9 content per se, so why not just crop to 4:3?
All modern screens are 16:9 and if you play it in full screen, there's no difference.I don't see a reason to do this, other than waste of space.
For example, playing it back on a 4:3 screen will end up with a "picture-in-picture" effect.
Do you know anyone who still has a TV from the 90ties or such an old computer monitor?
BTW: The original file also was in 1920x1080p.