Divinity: Original Sin

Divinity: Original Sin is a single-player and cooperative multiplayer fantasy role-playing video game by Larian Studios. The game ships with the editor that it was created with, allowing players to create their own single-player and multiplayer adventures, and publish them online. The game was funded through Kickstarter and is a prequel to Divine Divinity.

Gameplay

Original Sin is the latest RPG in the Divinity universe. Divinity: Original Sin concerns two heroes: a condemned warrior released from his chains and a mystic heroine restored to life. The game touts turn-based combat, cooperative multiplayer, a highly interactive world and will ship with a full-fledged modding tool.

Source: Wikipedia, "Divinity: Original Sin", available under the CC-BY-SA License.

There are unofficial mods which can make this a 4 or 6 player game