WK, Bungie is not developing Halo 4. After Halo: Reach, Bungie's contract with Microsoft expired. Now they have a ten-year deal with Activision; they're planning to announce their new title soon, I think, but if their recent job postings are any indication, it will be on the PS3.

Halo 4 (and the all future titles) are under the direction of 343 Industries (343i), which is made up of some Bungie employees who decided to stay with Microsoft to continue to work on the franchise.