Involved in anthocyanin and protoanthocyanidin biosynthesis by catalyzing the oxidation of leucoanthocyanidins into anthocyanidins. 2-oxoglutarate + a (2R,3S,4S)-leucoanthocyanidin + O2 = a 4-H-anthocyanidin with a 3-hydroxy group + CO2 + 2 H2O + succinate Binds 1 ascorbate molecule per subunit. Binds 1 Fe(2+) ion per subunit. Pigment biosynthesis; anthocyanin biosynthesis. Belongs to the iron/ascorbate-dependent oxidoreductase family.