This plant enzyme catalyses a step in the biosynthesis of sterols. It acts on the 4alpha-methyl group of the 4,4-dimethylated intermediate 24-methylidenecycloartanol and catalyses three successive oxidations, turning it into a carboxyl group. The carboxylate is subsequently removed by EC 126.96.36.1998, plant 3beta-hydroxysteroid-4alpha-carboxylate 3-dehydrogenase (decarboxylating), which also catalyses the epimerization of the remaining 4beta-methyl into the 4alpha position. Unlike the fungal/animal enzyme EC 188.8.131.52, 4alpha-methylsterol monooxygenase, this enzyme is not able to remove the methyl group from C-4-monomethylated substrates. That activity is performed in plants by a second enzyme, EC 184.108.40.206, plant 4alpha-monomethylsterol monooxygenase.