If you are you superimposing the 4x3 footage such that you still expect there to be 16x9 footage surrounding it, then there's nothing to it. Just put the 4x3 footage on a higher video track, make and motion tab adjustments you need/want to and you're done.
Now if you mean you want to cut 4x3 footage into a timeline and have it look like 16x9 footage, then you'll need to zoom into the footage and deal with the loss of quality and the like. but if its a dream sequence and you use some filters, it might not be too bad a hit for you. This is all in the idea that you're cutting an anamorphic timeline.
If you're cutting a regular 4x3 timeline, and just letterboxing the 16:9 footage shot, then you just need to do a minor zoom on the 4x3 footage, mask out the letterbox areas, do a bit of pan-n-scan perhaps, and voila.
So not too big a deal really. Just depends on what exactly you're trying to do, but either way its doable. But you will take a minor hit in quality one way or another as you'll need to do some degree of zoom in (or a mix of a slight zoom with some distortion, depending on image).
