Part of cytochrome c oxidase, its function is unknown. 4 Fe(II)-[cytochrome c] + 8 H(+)(in) + O2 = 4 Fe(III)-[cytochrome c] + 4 H(+)(out) + 2 H2O Associates with subunits I, II and III to form cytochrome c oxidase. The 4 subunit cytochrome c oxidase forms a supercomplex with cytochrome bc1. Belongs to the cytochrome c oxidase bacterial subunit CtaF family.