The Chief needs to return to Earth it seems, for it's the only way to fix Cortana. In the video above, taken from Halo Waypoint, 343 Industries teases where Halo 4 will be taking us–our very own home.
Cortana: I was put into service 8 years ago. A.I.'s deteriotate after 7, Chief.
Master Chief: If we can just get back to Earth and find Halsey, she can fix this.
Then Cortana advises the Chief to not make a promise he can't keep to a girl. Yikes! We might not know much of anything about Halo 4's plot yet–but Earth seems to be in its future.
Source: More Console