Glycosyl O-methyltransferase that catalyzes the final step in the biosynthesis of rebeccamycin, an indolocarbazole alkaloid that inhibits topoisomerase 1. Has broad substrate specificity and functions as glycosyl O-methyltransferase on a number of rebeccamycin analogs. 4'-demethylrebeccamycin + S-adenosyl-L-methionine = H(+) + rebeccamycin + S-adenosyl-L-homocysteine Optimum pH is 6.5-8. Monomer. Belongs to the methyltransferase superfamily.