I am attempting to use old analog footage, (640x480 Hi-8). I am trying to upscale to 1920x1080, with a 4:3 picture within a letterbox. While “square pixel” format helps, my finished image is still wider than 4:3, creating distortions.
you probably need to set the height/width or scale manually. The automatic caclulations seem a bit mysterious.
I am upscaling SD DVD rips to 4K and to get them to look properly proportioned I had to manually set the scale to 450% to get a 2880x2160 output (as well as use square pixel). Anything else resulted in output that was too wide (horizontally stretched). Simply using square pixels was closer that original, but still stretched. This resulted in true 4x3 output.
If you want to encode the pillar bars in the video (i.e. produce a 3840x2160 video) leave the scale at 450% and set the width to 3840.