Part of the mycobacterial fatty acid elongation system FAS-II, which is involved in mycolic acid biosynthesis. Catalyzes the NADPH-dependent reduction of beta-ketoacyl derivatives, the second step of the FAS-II elongation cycle. a (3R)-hydroxyacyl-[ACP] + NADP(+) = a 3-oxoacyl-[ACP] + H(+) + NADPH Lipid metabolism; mycolic acid biosynthesis. Homotetramer. Belongs to the short-chain dehydrogenases/reductases (SDR) family.