From Halopedia, the Halo wiki
Forge is a multiplayer mode featured in Halo 4. Unlike Halo: Reach, which featured a total overhaul of how Forge functioned compared to its Halo 3 incarnation, Forge in Halo 4 was based on the Reach version and did not feature so many drastic changes. Despite this, Halo 4 expanded on map creation features significantly. The mode was first shown off at RTX 2012.
Changes from Forge in Halo: Reach
Halo 4 with three Forge canvases, a first in the series history.
Following user feedback that the available Forge spaces were too restrictive for map-making, 343 Industries released the free Forge Island DLC for all players in March 2013, featuring a massive skybox and flat land for Forging in an environment very similar to that of Forge World in Reach.
Changes for Halo: The Master Chief Collection