Final answer:
The fourth ionization energy of zirconium cannot be determined without current data sources. Ionization energy usually increases from left to right on the periodic table.
Step-by-step explanation:
The question seeks to determine the fourth ionization energy of zirconium. Ionization energy is the energy required to remove an electron from a gaseous atom or ion.
The trend in ionization energies across the periodic table typically increases as we move from left to right due to a decrease in atomic radius and an increase in the electrostatic interaction between the nucleus and electrons.
However, listing specific ionization energies without having access to current data sources, such as a recent edition of the CRC Handbook of Chemistry and Physics, is not possible.
Thus, I am unable to provide a definitive answer to the question regarding zirconium's fourth ionization energy.
As for the information provided in the question, the first ionization energy of Mg is greater than that of Al, despite Al having a higher atomic number.
This can be explained by the fact that Mg is located at the end of its respective period with a full s-orbital, which is more stable and requires more energy to remove an electron. In contrast, Al starts a new p-orbital with only one electron, making it easier to remove.