I'm actually surprised this hasn't made it's way to the forum here yet. However there are coming more and more places I'm hearing this from and all credible but most still say it's a rumor.
The company Gearbox is rumored to be making Halo 4. For those that don't know Gearbox made the Halo:CE PC version, so if another company is going to make a Halo 4 other than Bungie they are a good choice I'd say.
Source (very reliable, written by and for game developers):
Gamasutra - New Rumors Connect Gearbox With Halo
More source (shows email written by Gearbox programmer):
Rumor: Why All This Gearbox Halo 4 Talk? Here's Why... (Maybe)
It's safe to say that another Halo franchise other than Halo Wars is being made but by who and what it is still hasn't been confirmed. Heck if you look at Microsoft's Job postings you can see a lot stating that they are hiring for a Halo franchise game.