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

The trimer provides a surface for interaction between the antigen-antibody complex and other complement components.

The alpha chain may be cleaved to release C4 anaphylatoxin, a mediator of local inflammation.

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,[8] can be of one of two types: C4A and C4B.