Catalyzes the NAD(P)(+)-dependent oxidative decarboxylation of the C4 methyl groups of 4-alpha-carboxysterols in post-squalene cholesterol biosynthesis. Also plays a role in the regulation of the endocytic trafficking of EGFR. a 3beta-hydroxysteroid-4alpha-carboxylate + NADP(+) = a 3-oxosteroid + CO2 + NADPH a 3beta-hydroxysteroid-4alpha-carboxylate + NAD(+) = a 3-oxosteroid + CO2 + NADH 4alpha-carboxyzymosterol + NADP(+) = CO2 + NADPH + zymosterone 4alpha-carboxy-4beta-methyl-5alpha-cholest-8-en-3beta-ol + NADP(+) = 4alpha-methyl-5alpha-cholest-8-en-3-one + CO2 + NADPH 4alpha-carboxy-5alpha-cholest-8-ene-3beta-ol + NADP(+) = 5alpha-cholest-8-en-3-one + CO2 + NADPH 4beta-methylzymosterol-4alpha-carboxylate + NADP(+) = 3-dehydro-4-methylzymosterol + CO2 + NADPH 4beta-methylzymosterol-4alpha-carboxylate + NAD(+) = 3-dehydro-4-methylzymosterol + CO2 + NADH 4alpha-carboxy-5alpha-cholest-8-ene-3beta-ol + NAD(+) = 5alpha-cholest-8-en-3-one + CO2 + NADH 4alpha-carboxy-4beta-methyl-5alpha-cholest-8-en-3beta-ol + NAD(+) = 4alpha-methyl-5alpha-cholest-8-en-3-one + CO2 + NADH 4alpha-carboxyzymosterol + NAD(+) = CO2 + NADH + zymosterone Steroid biosynthesis; zymosterol biosynthesis; zymosterol from lanosterol: step 4/6. Homodimer. Trafficking through the Golgi is necessary for ER membrane localization. Belongs to the 3-beta-HSD family.