If we consider each number cube on its own, we have a probability of 2/6=1/3 to get a number greater than 4: we have to roll either a 5 or a 6.
Since the two rolls are independent (i.e. the result of the first roll influences in no way the result of the second roll), if we want both rolls to return a number greater than 4, we have to multiply the probabilities:
So, each roll has a probability of 1/3 to show a number greater than 4, which implies that the probability of getting a number greater than 4 with both rolls is 1/9.