Catalysis of the reaction: (3,5-dichloro-2,4,6-trihydroxyphenyl)hexan-1-one + S-adenosyl-L-methionine = 1-(3,5-dichloro-2,6-dihydroxy-4-methoxyphenyl)hexan-1-one + H+ + S-adenosyl-L-homocysteine.
DIF-1 syntase