Catalysis of the reaction: S-adenosyl-L-methionine + 5,7,4'-trihydroxyflavone = S-adenosyl-L-homocysteine + 4'-methoxy-5,7-dihydroxyflavone.
S-adenosyl-L-methionine:5,7,4\'-trihydroxyflavone 4\'-O-methyltransferase activity