${\mathrm{Mg}=[\mathrm{Ne}] 3 \mathrm{s}^{2}}$
${\mathrm{Al}=[\mathrm{Ne}] 3 \mathrm{s}^{2} 3 \mathrm{p}^{1}}$
So order of first ionisation energy is
$\mathrm{Na}(496)<\mathrm{Mg}(737)>\mathrm{Al}(577)<\text { Si }{(786)}$
$\mathrm{Na}<\mathrm{Al}<\mathrm{Mg}<\mathrm{Si}\left(\mathrm{IE}_{1} \text { order }\right)$