Complement component 4 is a protein involved in the complement system.It is cleaved into proteins 4a and 4b.  C4b forms part of C3-convertase, in conjunction with 2a: C4b can bind CR1. C4b is the large fragment formed when Complement C4 is cleaved by Complement C1S. The membrane-bound C4b binds Complement C2A, a SERINE PROTEASE, to form C4b2a (CLASSICAL PATHWAY C3 CONVERTASE) and subsequent C4b2a3b (CLASSICAL PATHWAY C5 CONVERTASE).

In 3 black-American patients with SLE, Wilson and Perez (1988) found complete deficiency of plasma C4B. In a 9-year-old girl with SLE and complete C4 deficiency, Welch et al. (1990) found uniparental isodisomy 6. The girl had 2 identical chromosome 6 haplotypes from the father and none from the mother.