The rumor comes from OXM (Official Xbox Magazine) by way of VG247 and LMG. In the October issue of OXM, the rumor section of the rag points to 2013 for the next full, proper iteration in the Fable franchise.
Here's the quote for context…
"Look for the Fable cycle to continue with a proper, controller-based Fable IV in 2013, says The Ball."
Of course, the necessity to cite the "proper, controller-based" nature of Fable IV comes after the Fable title that appeared during Microsoft's lengthy and, quite honestly, boring E3 presentation centered around the Kinect. Our own Erik Norris is still bitter for having to wake up at 8AM to make the trip out for that show.
It was there that Fable: The Journey was shown. A decidedly not on-rails Kinect experience with the Fable branding. As decent as the game may have looked for Kinect users, more traditional Fable fans found it to be little more than a distraction.
If the rumor of Fable IV in 2013 is legit, then those cranky over The Journey may have a reason to settle down…slightly.