(In reply to limitations
B doesn't need to encode all the P(100, 4) possibilities, just the 4! = 24 arrangements of the selected 4 cards, as Jer has observed.
But even if B can use some kind of modulo 24 clue, the cards are shuffled before A gets them. So how does A know which is B's card?
Also, since the problem doesn't specify that B hands the cards to A, the magicians can't use that ingenious extra bit depending on the orientation of the pack.