The probability of a four of a kind in any given hand is 13*48/combin(52,5) = 0.0002401. Let’s assume in two hours you can play 120 hands. The probability of exactly two four of a kinds would be combin(120,2) × 0.00024012 × (1-0.0002401)118 = 0.000400095 = 1 in 2499.41.