The rumors were right: A new Fable game is in development at Playground Games, but it's not Fable 4.
Microsoft today announced a new Fable that will reboot the IP under a brand new developer. We don't know much about the game, but it could be a next-gen exclusive. The official Microsoft Store page says Fable is being developed for Xbox Series X and PC. The Xbox One family isn't listed in the supported platforms.

"Fable returns with a new beginning for the legendary franchise. Explore a land of fantastical creatures and wondrous places. Developed by Playground Games for Xbox Series X and Windows 10," reads a brief snippet from Microsoft.
No concrete info was revealed about the game's mechanics, but the brief teaser did hint the signature goofy humor is still intact. There's also more rumors the game is an MMO and could be built off the skeleton of the cancelled Fable Legends project that was in dev before Lionhead Studios was closed down.
If the new Fable does skip Xbox One, it may not release for another 2 years or so. Microsoft has said that all first-party games will release both on Xbox One and Xbox Series X, as well as Game Pass, for the next few years.