Complement component 4 (C4) is a protein involved in the complement system in human, primate, and other animals. It serves a number of critical functions in the complement system, including contributing component C4B, which bears a reactive thionolactone ring which covalently modifies the cell surfaces of infecting pathogens, and which is a part of the C3 convertase which readies the C3 protein for its role in the complement system. C4 defines the Chido/Rodgers blood group system, and it, and the gene encoding it, are being investigated for its possible role in schizophrenia risk and development.

Complement component 4 is responsible for the Chido/Rodgers blood group system. C4d may be a biomarker for systemic lupus erythematosus. The gene that codes for C4 is being investigated for the role it may play in schizophrenia risk and development.