One way………Move the cursor to the far left of the track. This adjust will effect everything on the track. Select ‘Track Motion’ from the track header. At the top of the Track Motion window, deselect ‘Lock Aspect Ratio’, then on the left under ‘Position’, adjust the ‘Width’ to the pixel size to equal an aspect ratio of 4:3 of the ‘Height’. When rendering adjust the pixel width and height to equal 4:3.
Best Regards……George