A mark IV top will NOT work on a mark III Sprite.
That is .. The later models that have a foldable top rather than a removable top are different enough that a hard top won't fit. There's a notch to fit the trim of the MK III Sprite that isn't there on the Mk IV.
As a matter of fact that's the only reason I have a hard top at all. The person I bought it from had completely restored a top. New glass, new gaskets, sanded repaired painted --perfect. Went to put it on and found that it was for Mk III Sprite (Mk II Midget). and put it up for sale. Lucky me.