Halo 4's iteration of the Forge map editor is currently under development at Certain Affinity, the development studio responsible for Age of Booty, Crimson Alliance and Halo: Combat Evolved Anniversary's multiplayer. The announcement was made during a presentation at Rooster Teeth's second annual RTX event in Austin, TX.
While a wide breadth of changes and additions have been made to the Forge editor, most interesting are the "Player Trait Zones," which allow map builders to modify the characteristics of any players that enter the effected area, changing the height of their jumps, the speed at which they run, etc. It's even possible to reverse the gravity in a given area by using "Grav Volume" field.
Enhancements have also been made to the objects that can be placed by users: Elements can now be locked in place, magnetized and more efficiently duplicated. The editor's lighting system has also been overhauled; objects placed now cast and receive shadows normally. Finally, a map editor capable of bringing our upside-down film noir Halo dreams to life.
Update: Now with video! Thanks, Austin!