Catalysis of the reaction: 3',4',5-trihydroxy-3,7-dimethoxyflavone + S-adenosyl-L-methionine(1+) = 3',5-dihydroxy-3,4',7-trimethoxyflavone + S-adenosyl-L-homocysteine + H+.
flavonol 4\'-methyltransferase activity