By separating one electron from an atom, the process of separating one electron from the obtained positive electric charge is relatively difficult.
By separating one electron from an atom, the radius of obtained positive electric charge is small and the attraction between its nucleus and external electron increases.
Hence the second ionization potential is greater than the first ionization potential.
Similarly, the third ionization potential is greater than the second ionization potential if aluminum has a greater difference between the fourth ionization potential and the third ionization potential.
The reason for this is that after the separation of three electrons from the aluminum atom, the number of cells in it decreases. Effective nuclear charge is increased as cell numbers decrease.
And the distance between the nucleus and external electrons decreases. Hence, the attraction and ionization potential increases between the nucleus and electron of the outer shell.