Is this going to shown on widescreen TVs? If so, you have 2 choices. Do you want A) the 4×3 video centered in the middle of the 16×9 frame so that it has black bars on the side or B) do you want the video blown up so that it fits a 16×9 frame, but loses some of the image and is soft from having been blown up?
If this video is going to be seen on 4×3 TVs, then you want to C) have a 4×3 project where the 16×9 footage is letterboxed.
You have to pick one of those choices, but once you do we can help you make it work.
(If this project could be seen on both kinds of TVs you may have to pick more than ONE of those choices and maket two different master versions, one for each type of viewing.)