Microsoft's 343 Industries has been working on Halo 4 for two years.
In a panel discussion at Comic-Con reported by GameSpot, Halo franchise boss Frank O'Connor said it was a relief to announce the 2012 Xbox 360 exclusive at E3 last month because the team has been working on it in "relative secrecy for a couple of years".
Halo's Covenant-beating hero will appear "changed" in Halo 4. And there's a story behind his new look yet to be revealed.
Some time has passed since the events of Halo 3, when the Chief finished the fight. Halo 4 opens with him floating through space towards an alien planet. "A lot of things have changed in that intervening period," O'Connor has teased.
Halo 4, out late 2012, follows 343 Industries' upcoming original Halo re-make - Halo: Combat Evolved Anniversary - out 15th November.
Anniversary includes a similar mechanic to Halo 3: ODST's terminals and data pads. The mechanic will provide insight into Halo 4 and extra backstory, with nutty AI Guilty Spark the subject.
Meanwhile, the panel discussed the upcoming Halo novels. Halo: Grasslands, due out in October, is set between Halo 3 and Halo 4, a time when Master Chief is adrift in space.