The answer is (2) 2. First of all, a greater hydronium ion concentration means more acidic, so we need a lower pH. Also, you need to remember that pH is on a base 10 log scale. This means that a difference by a factor of 10 in concentration is a difference of 1 on the pH scale. So since the difference is a factor of 100 (10^2), the difference is 2. We already established that the pH must be lower, so your answer is 4 - 2 = 2.