Ill be discussing the end of Halo 4 here, so expect some mild spoilers.
The Final ship on halo 4 that the Didact (thats his name right? the boss?) is on is designed if i remember correctly is designed to transform organic tissue into a A.I. or artificial material. We see this laser that is supposed to do this shoot and land on earth. I don't remember how many times, but here is my theory.
When Chief puts Cortana into the console and she starts splitting herself up, I believe Cortana came up with a plan to preserve herself. I believe she was able to figure out how to reverse the process of turning organic material into artificial material and turned her Artificial consiousness into a human on earth.
This opens up the possibilities for another theory... That cortana..... might actually suit up. What better way to show her return then have her save master chief in a spartan suit!