It is possible the determination cannot be made. If this 'real world' is limited to only the four cards, the statement could be shown not to be false. But one shot can sink the ship. If the first does, the second need not be assayed. In that case, "must you turn over" does not apply. Probabilites do not dictate the outcome. This is a conditional arrangement. It could have said, "Which cards should be selected to ensure the veracity of the statement can be determined?"