There have been rumors spreading that Halo 4 was to be a launch title for Microsoft's successor to the Xbox 360. That doesn't seem to be the case as it looks like Cheif is in for an extended stay.
Frank O'Connor, the development director for the Halo franchise, has just posted on Neogaf that Halo 4 will be on the 360.
"Halo 4 will be on Xbox 360 and use a modified core of the Halo engine(s). Not really a singular engine since it goes through fairly radical evolutions all the time."
I personally am not a Halo fan (didn't find the games very interesting) but I understand why so many people like it. This is also good news for a lot of people since most of them are probably not ready for a console move just yet. Or will have trouble convincing their parents to get them another Xbox.