Discovered on the LinkedIn profile of Paul Ehreth, designer for Microsoft's 343 Industries, is a list of his past and current projects. His most recent project, which is dated as July 2010 - present, lists that he's "designing new levels and missions for the next Halo project."
You have to assume that can only mean Halo 4, considering Halo: Reach is beyond the level and mission design stages and is due out in a couple of months - most likely not needing designers to come on board in July.
Additionally, Microsoft has been advertising their search for help on their site since the beginning of the month:
"343 Industries, Microsoft's internal Halo studio, is looking for an experienced Missions Designer who can design and build missions for our next big project."
This ad also solidifies that 343 Industries is the new developer behind the successful franchise, replacing Bungie who announced their independence from Microsoft, owners of the rights to the IP.
Bungie's community boss revealed that their last go with the Halo universe is ultimately Halo: Reach, but they did think about developing Halo 4 for their last hoorah:
"Even before the idea of a game based around Reach came about, a lot of other concepts were explored, up to and including a proper 'Halo 4,' where Master Chief was going to wake up from cryo-sleep and we were going to tell that story."
With a replacement developer already hitting the ground running, it looks like Microsoft is avoiding any lulls with the continuity of the IP. Although Bungie would have been the best candidate to continue the epic story of Master Chief, it will be interesting to see how another developer will do at the helm.
This also gives a chance for Bungie to use their talent to generate other IP's - we are anticipating the reveal of what their deal with Activision will bring.
What are your expectations for Halo 4? How do you feel about the story continuing without Bungie?