Minor envelope protein. Along with GP2a, serves as the viral attachment protein responsible for mediating interactions with CD163 thereby playing a role in virus entry into susceptible host cells (By similarity). Heterotrimer of GP2a, GP3, and GP4 (By similarity). The GP2a-GP3-GP4 complex associates with the E protein (Probable). Interacts with glycoprotein 5 (By similarity). Interacts with host CD163; this interaction plays a role in virus entry into host cell (By similarity). Only a small fraction of GP4 synthesized in infected cells ends up in virions. Belongs to the arteriviridae GP4 protein family.