Aspect ratio is governed by the timeline and the clips.
Rule 1: the aspect ratio of the first clip on the timeline whether video, still image, title or menu, governs all the following clips whatever their original a/r.
Rule 2: right clicking on any clip (of any sort) in the album will give you the option of 4:3 or 16:9.
If you've shot at both ratios on the same DV tape then you could banjax your production.
Capture 16:9 in project A and 4:3 in project B, merge the two in project C. Keep the w/s ration and edit out 4:3 clips which look absurd.
Breathe through your ears