Catalyzes the hydroxylation of L-4'-hydroxyisoleucine (4'-HIL) at the C-4 position to form L-4,4'-dihydroxyisoleucine (4,4'-DIHIL) (PubMed:23554367). Together with HilA, catalyzes the two step conversion of L-isoleucine into L-4,4'-dihydroxyisoleucine (PubMed:23554367). In vitro, in the absence of HilA, can also catalyze the oxidation of L-methionine and the C-4-hydroxylation of L-leucine and L-isoleucine (PubMed:22448874). 2-oxoglutarate + 3(1)-hydroxy-L-isoleucine + O2 = (4S)-3(1),4-dihydroxy-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.