The first ionisation enthalpies of Na, Mg, Al and Si are in the order:

Na configuration: 1s²,2s²,2p⁶,3s¹.

Mg configuration: 1s²,2s²,2p⁶,3s².

Al configuration: 1s²,2s²,2p⁶,3s²,3p¹

Si configuration: 1s²,2s²,2p⁶,3s²,3p²

As effective nuclear charge on Na is less than Mg, so the ionisation enthalpy of Na will be less than Mg as Mg has 2 electrons and Na has one electron in the outermost shell.

It is easier to remove an electron from p orbital than in s orbital so the ionisation enthalpy of Mg is less than Al. Therefore, Ionisation enthalpy of Mg is greater than Al.