Core protein 4a is the most abundant virion protein. Major component of the virion core that undergoes proteolytic processing during the immature virion (IV) to mature virion (MV) transition. Interacts with P39/A4. Probably localizes to the virion core wall. The precursor is cleaved by I7 to give rise to the 62 kDa 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). Belongs to the poxviridae protein P4a family.