Complement 4 or C-4 is a protein that moves freely in blood and helps in inflammation. Usually this
test is done to monitor response to management of autoimmune disease. C4 and other complements are low in lupus and high in rheumatoid arthritis and certain cancers. Decreased component activity is also seen in nephritis, cirrhosis, bacterial infections, malnutrition, hereditary angioedema, and hepatitis.
Hope this helps. Do let me know if there is any thing else and keep me posted. Take care!