Oh, and one more thing, Pan & Scan is not really intended to fit foreign aspect ratios into your sequence. It is intended as a top layer effect to change your show’s aspect ratio to another aspect ratio.
If you need to mix 4×3 and 16×9 footage, you should just set the Image Aspect Ratio flag in the bin (it’s in the columns). At the same time, you can determine how Avid should conform this source in the ‘Reformat’ column in the bin (Stretch, Letter/Pillarbox, etc.).
If you do that, all shots should look correct as soon as you open them in source or record monitor. Also, you can add other effects at will.