Reduces the C14=C15 double bond of 4,4-dimethyl-cholesta-8,14,24-trienol to produce 4,4-dimethyl-cholesta-8,24-dienol. Required for cell division and expansion and is involved in proper organization of the embryo. 4,4-dimethyl-5alpha-cholesta-8,24-dien-3beta-ol + NADP(+) = 4,4-dimethyl-5alpha-cholesta-8,14,24-trien-3beta-ol + H(+) + NADPH Steroid biosynthesis; zymosterol biosynthesis; zymosterol from lanosterol: step 2/6. A number of isoforms are produced. According to EST sequences. Belongs to the ERG4/ERG24 family.