A period 4 element is an element in the fourth period (row) of the periodic table. It begins at potassium and ends at krypton. Period 4 has 18 elements.

Period 4 elements have electrons in the first four electron shells. All period 4 elements have one or more electrons in the fourth electron shell (valence electrons). When atomic number increases by one, another electron is added. Two electrons join the 4s orbital, ten electrons join the 3d orbital, and six electrons join the 4p orbitals.