Thanks very much for the response!
However, I just tried this and now my footage is stretched from 4:3 to 16:9, and, at 100%, covers the 4K (3840×2160) frame area. I can only guess that the program is interpreting it as anamorphic somehow? at 100% it should be larger than the frame..
Unfortunately I am still unclear on how to fix this without re-rendering out the footage at the standard resolution…