Halo 4, and now we know what she looks like (care of the latest Game Informer cover reveal, as seen above).
Given the conclusion of Halo 3, we were kinda expecting the Chief's blue buddy to have a straitjacket wrapped around her projected avatar, but perhaps that and more will be explained when the game arrives later this year.
Oh, and for comparison's sake, we've dropped an image of Halo 3's Cortana just below the break. Her newer likeness is kinda ... less blue? A bit? That's all we got.