Minor envelope protein. Part of the glycoproteins heterotrimer GP2b-GP3-GP4 which is probably responsible for the attachment to target host cell. This attachment induces virion internalization predominantly through clathrin-dependent endocytosis. Heterotrimer of GP2b, GP3, and GP4; disulfide-linked (Probable). The GP2b-GP3-GP4 complex associates with the E protein (By similarity). Interacts with glycoprotein 5 (By similarity). Only a small fraction of GP4 synthesized in infected cells ends up in virions. Belongs to the arteriviridae GP4 protein family.