Component of the COP9 signalosome (CSN) complex that acts as an regulator of the ubiquitin (Ubl) conjugation pathway by mediating the deneddylation of the cullin subunit of SCF-type E3 ubiquitin-protein ligase complexes. Component of the COP9 signalosome (CSN) complex composed of at least csn1, csn2, csn4 and csn5 subunits. Belongs to the CSN4 family.