So 2 players can share the same screen, and a further 2 players can join them via Online.
Launch the game on the host account, and have the secondary controller press start to begin playing.In order to play split-screen, players will need to have two controllers, as you cannot play with keyboard/mouse for one person and controller for another person.Local co-op (Couch co-op) is capped at 2 players.Please note there's no cross-platform multiplayer: Those playing on PC cannot play with PS4 or Xbox One Players, and PS4 and Xbox One cannot cross-play across platforms either. Players may enjoy split-screen 'couch co-op' or join online sessions from other players, or any combination therein. Multiplayer is available in Divinity: Original Sin 2 in both online and offline versions.