ORF4 polyprotein codes for VP4a, VP4b, and VP4c, three of the four proteins that self-assemble to form the icosahedral capsid. The capsid is made of VP3 (coded by ORF3), VP4a, VP4b and VP4c. Proteolytic processing of ORF4 polyprotein yields the VP4a, VP4b and VP4c capsid proteins.