Why is there a difference between the way orbitals are filled and electrons are removed?

I know that when determining the order in which orbitals fill, you use Aufbau principle (1s^2, 2s^2, 2p^6, 3s^2, ect.) but my teacher told me that when dealing with anions in the d and f orbitals, the electron must be removed from the highest orbital in numerical order (1s^2, 2s^2, 2p^6, 3s^2, 3p^6, 3d^10, 4s^2) meaning that the electron should be removed from the 4s^2. one of the questions on my homework is what atomic orbital is filled just before the 5f orbitals and I am confused as to what the answer should be.