Catalysis of the reaction: 4,4-dimethyl-5alpha-cholest-7-en-3beta-ol + 6 Fe(II)- + 5 H+ + 3 O2 = 4alpha-carboxy-4beta-methyl-5alpha-cholest-7-ene-3beta-ol + 6 Fe(III)- + 4 H2O.
4-methylsterol oxidase activity