If for some reason you want your widescreen footage exported to 4:3 (eg. sometimes festivals require 4:3), I think your options are with the camera, not PP. When capturing footage, with my camera anyway, I can choose to downconvert during capture through either squeeze, edge crop or vertical bars. I don't think you can choose through PP - once you've chosen 4:3, it just accepts whatever you've set up via the camera.