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. Localizes to the virion core wall, the mature protein accounts for 11% of the dry mass of the virion. The 73-kDa precursor is cleaved to a mature protein of 60 kDa 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). Belongs to the poxviridae protein P4b family.