Miya-san

If You think the Classic Complement pathway must start with the activation of C4 and C2 to form a C3-convertase (C4b2a), it will be necessary the presence of C4. So, if C4 levels are low, probably C3 levels of Classic pathway remains low. The Lectin Complement pathway needs the C4 and C2 to form a C3-convertase (C4b2a) too, so it will be the same way of Classic Pathway. But Alternative Complement Pathway can use C3 convertases from Classic and Lectin pathway and also occur an hydrolysis of C3 protein, there's no need of C4. If complements are low, this imply that the person will be more susceptible to bacterial and viral infections. Think about opsonin and anaphylatoxin on pathogen surface and on plasma when the complement pathway is activated. Receptors of phagocytes cells would recognize those peptides. If You have less complements, it will be less opsonin and anaphylatoxin, more bacteria will survive and not be destroyed. That's why people with deficiency of complement (and its proteins) are more susceptible to bacterial reinfection.