"Work on the game started actually before we launched Halo 4 and continues apace as we cycle staff through the team's vacation breaks," development director Frank O'Connor said. "A lot of people at 343 are already prototyping ideas and software and art for future Halo products."
Speaking to CVG, O'Connor wasn't able to confirm if the next game is a "Halo 5," but that seems likely.
In addition to the next game, 343 is currently working on the Crimson Map Pack for Halo 4, and will continue working on the first season of Spartan Ops. Like a TV show, the season is taking hiatus until January. Episodes 1-5 will again be available (one a week) from December 17 through January 14 for anyone who missed them.