Four letters are radically different: H (the ASL '8'/'horns' handshape), J (a swiveling Y rather than I), X (uses two fingers, like a flexed ASL V), and T (just like the French F, but with the thumb on the inside of the index finger instead of on the outside).
There are notable differences in the way four letters are represented: H is formed using the ASL '8'/'horns' handshape, J involves a rotating Y motion, X uses two fingers like an ASL V, and T has a thumb placement similar to the French F.