When factorials are involved, leave your answer in terms of factorials (e.g., 5!/(3! ⋅ 2)).
Hint: To calculate the probability that exactly 4 cards are dealt until two Aces are shown, consider the fact that the first Ace could be the first card, the second card or the third card. The fourth card is, of course, the second Ace. Then no more cards are dealt.