Forge is a returning feature in Halo 4 and has been upgrade in several ways that should make map creation significantly easier for builders of all levels while granting some new controls that didn't previously exist.
The first thing you will notice is that objects in Forge will light up and reveal the name of them whenever you put your cursor over them. When you highlight them, they will turn green and when you pick it up, it will change from green to yellow.
After putting objects in their place, you can lock them into position so that they won't move accidentall unless you unlock them. Like the other glowing effects, an object that is locked will glow red, showing that it has been locked.
If there is an object on the screen that you want to make another of, there is a method of "duplicating" them. What characteristics are transfered upon duping is unknown however it will likely result in the object retaining the same rotational position and any coloring changes (if any are possible).
If you have an object placed and you want to put another object right up against it to create a seamless transition, they've added a magnet feature that will cause edges to line up prefectly so you don't have to keep moving them to get it flush or phase objects through each other.
Further, the ability to create a Player Trait Zone where you can alter player traits such as gravity, shields and so on.
All objects in forge have been recreated to be more detailed and better looking. They have also been added to the lighting system so that they can be shadowed or cast shadows as appropriately.