Halo 4 was being secretly developed for the last 2 years

Microsoft's 343 industries has been secretly working on Halo 4 for the last 2 years. The studio which was established in 2007 by Microsoft after Bungie decided to part ways with them in order to become an independent studio.

The announcement was made during the Halo-centric Comic-Con 2011 panel which was kicked off by Franchise Director Frank O'Connor(moved from Bungie to 343 after the split) who said,

It was a relief to have finally announced Halo 4, given that the team at 343 has been working on it in relative secrecy for a couple of years.

He later on discussed about the upcoming books which aren't necessary to understand the story, but will definitely enhance the story experience for the fans. The first book Halo: Grasslands will be released this October which bridges the gap between Halo 3 & 4's time frame, where Master Chief is adrift in space for an indeterminate amount of time.

343 industries is also working on Halo: Combat Evolved Anniversary edition which is looking absolutely gorgeous with the graphical upgrade.