This difference is strong evidence that aluminum has three valence electrons, a predicted by quantum mechanics.
If the first three IEs are relatively low, as they are for Al, it indicates that the three electrons are each being removed from the valence shell (the third shell in this case). Since they would have fairly high energy in this shell, only a small additional amount is required to remove them.
However, the fourth electron to be removed must come from the second shell. In this shell, it is at a far lower energy to begin with, being close to the strong pull of the nucleus (with little shielding from the other electrons). Thus, we must provide a great deal of energy to remove it from the atom.