Catalyzes the chiral reduction of acetoacetyl-CoA to (R)-3-hydroxybutyryl-CoA. Is involved in the biosynthesis of polyhydroxybutyrate (PHB), which is accumulated as an intracellular energy reserve material when cells grow under conditions of nutrient limitation. a (3R)-3-hydroxyacyl-CoA + NADP(+) = a 3-oxoacyl-CoA + H(+) + NADPH (3R)-3-hydroxybutanoyl-CoA + NADP(+) = acetoacetyl-CoA + H(+) + NADPH kcat is 102 sec(-1). Biopolymer metabolism; poly-(R)-3-hydroxybutanoate biosynthesis. Homotetramer. Belongs to the short-chain dehydrogenases/reductases (SDR) family.