How do you get FFMPEG to change the aspect ratio from 16:9 back to 4:3 but NOT letterbox (unstretch)

This is the opposite of the question that is usually posted. Many Youtube videos that claim to be "remastered" versions of old films are incorrectly rendered as 16:9 instead of 4:3 and are stretched so that circles appear to be ovals. This can be corrected by changing the aspect ratio in VLC from 16:9 to 4:3, but how can the original video be remuxed or reencoded so that it is restored to its correct and original shape?

Simply, scale it back to 4:3 and set a unit aspect ratio.

ffmpeg -i in -vf scale=ih*4/3:ih,setsar=1 -c:a copy out.mp4

Try -aspect, which avoids re-encoding the video:

ffmpeg -i "$in_file" -c copy -aspect 4:3 "$out_file"