Catalyzes the hydroxylation of L-isoleucine to produce (4S)-4-hydroxy-L-isoleucine (PubMed:20665018, PubMed:19850012, PubMed:21821743). Can also catalyze the hydroxylation of L-leucine, L-norvaline, L-norleucine and L-allo-isoleucine, as well as the sulfoxidation of L-methionine, L-ethionine, S-methyl-L-cysteine, S-ethyl-L-cysteine, and S-allyl-L-cysteine (PubMed:21821743). 2-oxoglutarate + L-isoleucine + O2 = (4S)-4-hydroxy-L-isoleucine + CO2 + succinate Binds 1 ascorbate molecule per subunit. Binds 1 Fe(2+) ion per subunit. 4-Hydroxyisoleucine is a natural nonproteinogenic amino acid that exhibits insulinotropic biological activity and increases glucose-induced release of insulin. Belongs to the iron/ascorbate-dependent oxidoreductase family.