Complement component 4 is a protein involved in the complement system.
It is cleaved into proteins 4a and 4b; 4b can be further cleaved into 4c and 4d.
It is responsible for the Chido Rodgers blood group system.
C4d has been identified as 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.