In the film "Halo: Legends", one story showed that, after the events of Halo 3, Cortana talked to John-117. Near the end of the short story, it is seen by the audience that Cortana is headed closer and closer to rampancy, due to isolation rather than aging. However, as seen in the Halo 4 trailer, Cortana was seen live and well. My best guess is that Cortana was starting to experience the effects of isolation with a little bit of rampancy. However, she has not gone completely rampant and might only be a little rampant. Since "Smart AIs" are supposed to live for seven years, Cortana should stay in service until 2556, since she started her service in 2549.