Catalysis of the reaction: (S)-mandelate + 5,6,7,8-tetrahydrobiopterin + O2 = (S)-4-hydroxymandelate + 7,8-dihydrobiopterin + H2O. (S)-2-hydroxy-2-phenylacetate is also known as S-mandelate.
L-mandelate 4-hydroxylase activity