Non-enzymatic component of C3 and C5 convertases and thus essential for the propagation of the classical complement pathway. Covalently binds to immunoglobulins and immune complexes and enhances the solubilization of immune aggregates and the clearance of IC through CR1 on erythrocytes (By similarity). Derived from proteolytic degradation of complement C4, C4a anaphylatoxin is a mediator of local inflammatory process. It induces the contraction of smooth muscle, increases vascular permeability and causes histamine release from mast cells and basophilic leukocytes. This protein is synthesized as a single-chain precursor and, prior to secretion, is enzymatically cleaved to form a trimer of non-identical chains alpha, beta and gamma. C4 is a major histocompatibility complex class-III protein.