Tail protein located at the vertex occupied by the portal ring. 12 copies of gp4 form a structural ring at the bottom of the portal ring. Together with gp10 and gp26, gp4 is required for stabilization of the condensed DNA within the capsid; perhaps by plugging the hole through which the DNA enters. Probably functions as a head completion protein. Plays a role in ejection of the bacteriophage DNA into the host cell at the initiation of infection. Functions as an exolysin that catalyzes the cleavage of the glycosidic bonds between N-acetylmuramic acid and N-acetylglucosamine residues in peptidoglycans. Monomer; homododecamer upon binding to the portal ring (PubMed:16970964). Interacts (via C-terminus) with the portal protein; this interaction participates in the head completion (PubMed:28134243, PubMed:16970964). The N-terminus changes its conformation when transitioning from the preassembly state to the postassembly state.