I know the sprockets are different, but are the metal tracks the same for both?

I only have the PzIII. But, near as I can tell from all of the pictures, they are the same tracks. However, I think I read somewhere that the PzIV will have a few more links than the PzIII.

They are identical - I have plastic and metal tracks for both Panzer III's and Panzer IV's.

Yes they are the same, but the IV obviously has more links!