Why the energy difference between the removal of the 2nd and 3rd electrons is smaller than that between the removal of the 3rd the 4th electrons ?

Let us remember the fact that electrons actually repel each other in the atom. The more the number of electrons, the greater the repulsion experienced.

Hence, as electrons are successively removed from the atom at each stage of ionization, the magnitude of repulsion due to the presence of more electrons decreases as further electrons are removed, ionization energy consequently increases. Thus, the energy difference between the removal of the 2nd and 3rd electrons is smaller than that between the removal of the 3rd the 4th electrons.

Sometimes, when the removal of the 4th electron has to occur in an inner filled shell which is more stable, a far higher amount of energy is required (a quantum jump) compared to the removal of any of the first three electrons that are present in an incompletely filled outermost shell, E.g in aluminium.