P4 proteins are hexameric RNA packaging ATPases of dsRNA bacteriophages of the Cystoviridae family. P4 hexamers are integral part of the inner polymerase core and play several essential roles in the virus replication cycle. P4 proteins are structurally related to the hexameric helicases and translocases of superfamily 4 (SF4) and other RecA-like ATPases. Recombinant P4 proteins retain their 5' to 3' helicase and translocase activity in vitro and thus serve as a model system for studying the mechanism of action of hexameric ring helicases and RNA translocation. This review summarizes the different roles that P4 proteins play during virus assembly, genome packaging, and transcription. Structural and mechanistic details of P4 action are laid out to and subsequently compared with those of the related hexameric helicases and other packaging motors.