Major component of the virion core that undergoes proteolytic processing during the immature virion (IV) to mature virion (MV) transition. Essential for the formation of a structurally normal core (By similarity). Localizes to the virion core wall. The precursor is cleaved to a mature protein during virion maturation. Proteolytic cleavage of major core proteins P4a (A10L), P4b (A3L), and VP8 (L4R), which occurs at a late stage of core formation, is required for production of infectious mature virions (MV) (By similarity). Belongs to the poxviridae protein P4b family.