Catalysis of the reaction: 3-beta-hydroxy-4-beta-methyl-5-alpha-cholesta-8,24-dien-4-alpha-carboxylate + NAD(P)+ = 4-alpha-methyl-5-alpha-cholesta-8,24-dien-3-one + CO2 + NAD(P)H.
C-3 sterol dehydrogenase (C-4 decarboxylase) activity