By now we all seen the true ending but i got info on Halo 4.
HELLO HALO 4! You pretty only much know what's going on unless you
have read the books, OR have a magazine that summarizes everything
for you, YES!
*SPOILER WARNINGS!*
What happens is Master Chied's half-ridden ship is floating towards
a black planet.
The black planet's name is Onyx. And yes, there are other Spartans,
not just John, or Master Chief. Their names are Fred, Kelly and
Linda. They're all trapped on Onyx - insied a "fallout shelter" of
sorts that's designed to protect its occupants in the event of the
Halos being activated - but they're alive.
Also, Cortana is not the only learning A.I. construct. She and
others were created by the UNSC's Dr. Catherine Halsey,
who is traped with the remaining Spartans on Onyx.
Dr. Halsey made Cortana in her own image. Not too hard to notice...
Big help from - Xbox Official Magazine (XBOM)