The game takes place after the events of A Crack in Time and the Comic Book series, and follows main characters Ratchet, Clank, Captain Qwark and Doctor Nefarious, respectively the protagonist, deuteragonist, tritagonist and antagonist of the series. The game takes place when an evil plan set in motion by Nefarious to destroy his three archnemeses once and for all backfires, sending the four of them into the snare of a mysterious machine known as the Creature Collector. Now, they are forced to work together and cooperate in order to escape from the machine and get back home.
Unlike previous games in the series which were mostly single-player only games, All 4 One focuses on a four player cooperative multiplayer mode which allows for drop-in and drop-out online as well as offline multiplayer. Players can each take the role of one of the four main characters of the game namely Ratchet, Clank, Qwark and Doctor Nefarious.
Source: Wikipedia, "Ratchet & Clank: All 4 One", available under the CC-BY-SA License.