Catalysis of the reaction: S-adenosyl-L-methionine + 3'-hydroxy-N-methyl-(S)-coclaurine = S-adenosyl-L-homocysteine + (S)-reticuline.
S-adenosyl-L-methionine:3\'-hydroxy-N-methyl-(S)-coclaurine 4\'-O-methyltransferase activity