Catalyzes the methyl transfer from S-adenosyl-methionine to the C-4 of the A-ring sterols such as lathosterone (5alpha-cholest-7-en-3-one) thereby rendering them unsuitable as ligand precursors. May irreversibly shunt sterols away from hormone dafachronic acid production. Dafachronic acids act as ligands and bind directly to the nuclear hormone receptor (NHR) daf-12 suppressing dauer formation and inducing reproductive growth. By reducing the biosynthesis of dafachronic acids, this methyltransferase can regulate dauer larva formation. 5alpha-cholest-7-en-3-one + S-adenosyl-L-methionine = 4alpha-methyl-5alpha-cholest-7-en-3-one + H(+) + S-adenosyl-L-homocysteine Steroid hormone biosynthesis; dafachronic acid biosynthesis. Expressed in the pharynx and hypodermal syncytium. Mutants show impaired dauer larvae production. Belongs to the class I-like SAM-binding methyltransferase superfamily. Erg6/SMT family.