In period 4 of the periodic table, the elements with highest and lowest atomic radii are respectively.

In period 4, atomic radii decrease from left to right as nuclear charge increases while electron shell remains constant. K (potassium) is at the extreme left of period 4 metals and has the highest atomic radius. Br (bromine) is near the right end as a nonmetal and has the lowest atomic radius in this period. Therefore, the elements with highest and lowest atomic radii in period 4 are K and Br respectively.