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 (By similarity). Complex I is composed of 45 different subunits. This is a component of the iron-sulfur (IP) fragment of the enzyme (By similarity). Belongs to the complex I NDUFS4 subunit family.