Hey Toino
Split screen is only for two players on PC (do not know if on consoles but I think it is same) - Idea about that co-op added 3rd a 4th player is about that in DOS 1 was only for two players and actualy is possible to connect online another two.
And I think you are wrong with that image on website, because this there are two images which shows multiplayer - this one is about split screen: https://ibb.co/d3Tdg5
You can also see in this devlog: https://www.kickstarter.com/projects/larianstudios/divinity-original-sin-2/posts/1947157
