First make a quality Mpeg of your 4:3 project.
Then what I do,
is start a NEW 16:9 project,
and then import the 4:3 project.
Import into a different bin to your new project so as not to mess up sequences and so on.
From now on stick with the 16:9 project.
Be careful about your labeling so you know what is what.
Now, Get the 4:3 project into the timeline and arrange it best by pixel formations { down here we convert pixels to 1.33 for 720 by 576 ]
or alternatively within “Effects control” or standard effect “Motion” [ switch of scale control ] and change the scale so the 4:3 Mpeg fits in the way you want.
Sometimes I make photoshop frames in yellow colours to check the dimensions .
Then render and check it out.
It should work, I have to do it all the time.
paulscottfilms