Involved in the biosynthesis of the anthracyclines carminomycin and daunorubicin (daunomycin) which are aromatic polyketide antibiotics that exhibit high cytotoxicity and are widely applied in the chemotherapy of a variety of cancers. In vivo, catalyzes the transfer of a methyl group from S-adenosyl-L-methionine to the 4-O-position of carminomycin to form daunorubicin. In vitro, it also methylates the anthracyclines rhodomycin D (10-carbomethoxy-13-deoxycarminomycin) and 13-deoxy-carminomycin at the 4-hydroxyl position. It is quite specific with respect to the length of the carbohydrate chain at the C7 position, but it can accept substrates with bulky substituent at C10 position. carminomycin + S-adenosyl-L-methionine = daunorubicin + H(+) + S-adenosyl-L-homocysteine Antibiotic biosynthesis; daunorubicin biosynthesis. Antibiotic biosynthesis; carminomycin biosynthesis. Homodimer and homotetramer in equilibrium. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family.