Lionhead Studios have said there are no plans for a Fable IV.
Before he left Lionhead, Peter Molyneux, the former creative director for the series, expressed interest in creating a fourth game. He stated "Forget about the word 'trilogy.' I'm not going to tell you what happens at the end of Fable 3 but I don't think you'll feel in any way that you've reached the end of a trilogy."
There is also a game called Fable: The Journey which is a first-person shooter; however this is not Fable IV and is only available for Kinect. There is the remake of the original Fable called Fable Anniversary and is only available on Xbox 360. Finally, there is Fable Legends, an online action/strategy game set in Albion's past. This is not Fable IV either and will only be available on Xbox One.