for I4, we begin removing an electron with n = 2. For I3, we remove an electron with n = 3 (the last valence electron). In going from n = 3 to n = 2, there is a big jump in ionization energy because the n = 2 electrons are closer to the nucleus on average than the n = 3 electrons. Since the n = 2 electrons are closer, on average, to the nucleus, they are held more tightly and require a much larger amount of energy to remove compared to the n = 3 electrons. In general, valence electrons are much easier to remove than inner-core electrons.