Catalyzes the hydroxylation of L-isoleucine at the C-4' position to form L-4'-hydroxyisoleucine (4'-HIL) (PubMed:23554367). Exhibits low activity with L-valine and L-methionine (PubMed:23554367). 2-oxoglutarate + L-isoleucine + O2 = 3(1)-hydroxy-L-isoleucine + CO2 + succinate Binds 1 ascorbate molecule per subunit. Binds 1 Fe(2+) ion per subunit. Belongs to the iron/ascorbate-dependent oxidoreductase family.