Catalysis of the reaction: acetyl-CoA + 3 H+ + 3 malonyl-CoA + NADPH = 6-methylsalicylate + 3 CO2 + 4 CoA + H2O + NADP+.
acyl-CoA:malonyl-CoA C-acyltransferase (decarboxylating, oxoacyl-reducing, thioester-hydrolysing and cyclizing)