Acts in the assembly and extrusion of the bacteriophage by forming a channel across the host outer membrane. This channel is just large enough to allow a newly synthesized phage particle to pass through. Extrusion is a process of concomitant assembly and secretion and takes place at specific assembly sites where host inner and outer membranes are in close contacts. Homomultimer. The channel is composed of 14 G4P subunits that confer a barrel-like structure. Interacts with G1P; this interaction results in a complex that spans the inner an outer host membranes (By similarity). Belongs to the inovirus G4P protein family.