First question is do you want to end up with a 16:9 video or a 4:3 video?
If the later I'd just crop the 16:9 to 4:3. To do this put everything into a 4:3 project, make certain the 16:9 footage is flagged as 16:9. Then use event pan/crop to cutout a 4:3 frame from the 16:9 frame. You can set the control widget so you can only slide the crop sideways. This makes it easy to pan the crop if you need to adjust the framing.