[5] This gene encodes the acidic form of complement factor 4, part of the classical activation pathway.

Deficiency of this protein is associated with systemic lupus erythematosus and type I diabetes mellitus.

[6][7][8][9][10][11] Excess production due to a copy number that is higher than normal has shown a high probability of a causal relationship with schizophrenia and bipolar disorder with psychosis, which could explain the hereditary nature of these illnesses.

[12] This gene localizes to the RCCX locus within the major histocompatibility complex (MHC) class III region on chromosome 6.

[5] Each copy of the gene, due to five adjacent nucleotide substitutions cause four amino acid changes and immunological subfunctionalization,[15] can be of one of two types: C4A and C4B.