Catalyzes the transfer of the methyl group to the 4'-hydroxyl group of 3'-hydroxy-N-methylcoclaurine to form reticuline. (S)-3'-hydroxy-N-methylcoclaurine + S-adenosyl-L-methionine = (S)-reticuline + H(+) + S-adenosyl-L-homocysteine Alkaloid biosynthesis; (S)-reticuline biosynthesis; (S)-reticuline from (S)-norcoclaurine: step 4/4. Homodimer. Belongs to the class I-like SAM-binding methyltransferase superfamily. Cation-independent O-methyltransferase family. COMT subfamily.