You can try this, it worked for me.
After synching up 4:3 & 16:9 tracks, create mulicam track and make cuts using multicam mode.
Then click “expand to mulitple tracks” (in same menu, right under “create mutlicam track”)
You will be asked if you want to “Keep unused takes as muted events”.
I selected no and the tracks were split up in timeline.
I then was able to fill 16:9 screen with the 4:3 track using the “Track Motion” button (above level slider in track control)
Remember to hold ctrl key while selecting all clips you want to expand so all 4:3 clips fill 16:9 screen the same.
I was able to add fades, etc. to each clip. Remember to uncheck ripple edit so the clips stay synched if you decide to increase or decrease any clip’s length.