Catalyzes the NAD(P)(+)-dependent oxidative decarboxylation of the C4 methyl groups of 4-alpha-carboxysterols in post-squalene cholesterol biosynthesis. a 3beta-hydroxysteroid-4alpha-carboxylate + NADP(+) = a 3-oxosteroid + CO2 + NADPH a 3beta-hydroxysteroid-4alpha-carboxylate + NAD(+) = a 3-oxosteroid + CO2 + NADH Steroid biosynthesis; zymosterol biosynthesis; zymosterol from lanosterol: step 4/6. Homodimer. Belongs to the 3-beta-HSD family.