Accessory subunit of the mitochondrial membrane respiratory chain NADH dehydrogenase (Complex I), that is believed not to be involved in catalysis. Complex I functions in the transfer of electrons from NADH to the respiratory chain. The immediate electron acceptor for the enzyme is believed to be ubiquinone. Mammalian complex I is composed of 45 different subunits. This is a component of the iron-sulfur (IP) fragment of the enzyme. Interacts with BCAP31 and TOMM40; the interaction mediates its translocation to the mitochondria; the interaction with BCAP31 is direct. The interaction with BCAP31 mediates mitochondria localization. Belongs to the complex I NDUFS4 subunit family.