Since magnesium and calcium have the same number of valence electrons, and since the valence electrons are located in the outermost shell so the reasonable is 4
Magnesium and Calcium both have 2 electrons in their valence/outermost shell. Chemical properties of an element is determined by the configuration of its electrons in orbits around nucleus. Since the number of protons and electrons are the same it essentially means atomic number determines the chemical behavior of an element so that is why Mg and Ca have same chemical properties.
