The Sims 4 Mod Support will come right out of the box, built in, from day 1. The game will come pre-prepared with a Mods folder, instructional documentation, modding forums, and mod options menus. Previously, EA did not support any mods on Sims games, and this is now changing. They will be taking a different route with expansions and DLC, which we do not yet know about. Modders will be able to change anything in the game that they can manage, using the coding language Python. Python is what The Sims 4 is built on, and is fairly easy to use.
EA states that the community makes awesome stuff, and really wants them to be able to have a user-friendly way to implement their creations. The problem in the past was they didn’t have the time or resources to go through and filter what was good and bad, so they’ve decided to give the user full control. The Sims is already an open world game, but a whole new world has just been opened up.