Component of the heterotetramer complex KAR (3-ketoacyl-[acyl carrier protein] reductase or 3-ketoacyl-[ACP] reductase) that forms part of the mitochondrial fatty acid synthase (mtFAS). Beta-subunit of the KAR heterotetramer complex, responsible for the 3-ketoacyl-ACP reductase activity of the mtFAS, reduces 3-oxoacyl-[ACP] to (3R)-hydroxyacyl-[ACP] in a NADPH-dependent manner with no chain length preference, thereby participating in mitochondrial fatty acid biosynthesis. The homotetramer has NADPH-dependent quinone reductase activity (in vitro), hence could play a role in protection against cytotoxicity of exogenous quinones. As a heterotetramer, it can also reduce 9,10-phenanthrenequinone, 1,4-benzoquinone and various other o-quinones and p-quinones (in vitro). a (3R)-hydroxyacyl-[ACP] + NADP(+) = a 3-oxoacyl-[ACP] + H(+) + NADPH a quinone + H(+) + NADPH = a quinol + NADP(+) Lipid metabolism; fatty acid biosynthesis. Homotetramer (in vitro). Heterotetramer with HSD17B8; contains two molecules each of HSD17B8 and CBR4. Does not form homotetramers when HSD17B8 is coexpressed, only heterotetramers (in vitro). Belongs to the short-chain dehydrogenases/reductases (SDR) family.