The Fourth, or IV, was one of Rome's Elite Legions in Ryse: Son of Rome.
The Fourth is posted in Britannia at some point before the events of Ryse. Weather or not they campaigned against barbarians before the events of Ryse is unclear.
At some point during the early stages of King Oswald's rebellion, the Fourth moved to defend a Roman aqueduct that was being constructed in the shadow of the hill where Oswald's throne resided, and engaged Oswald's forces throughout the surrounding forest. The Fourth was completely annihilated during the battle, and the Romans working on the aqueduct were massacred by the Britons.
In Chapter 4, while infiltrating Oswald's camp in an attempt to capture him alive, Marius Titus discovers the remains of the Fourth Legion mutilated and left on display throughout the forest around the aqueduct as a grizzly warning to ward off future Roman incursions.
Marius acknowledges that several of the corpses he discovers were killed by traps, and due to the dense and thick nature of the forest, it is clear that the Britons ambushed the Fourth and slaughtered them in the cramped environment. This was a common tactic used against Rome's Legion's by barbarian forces throughout history, as it prevented the Roman's from forming and maintaining tight-formation, which was the cornerstone of Rome's military strategy.