This is possible with D:OS 2. In D:OS EE, officially there is only 2 player multiplayer. Mods can allow up to four players, and there is a mod to enable 4 character creation (a third player could take over a companion character after you get to town and recruit one or more, but companions can not speak to NPCs except to trade).
For LAN co-op in D:OS EE, have the host go into the Game section of the options, and enable LAN connections (and possibly Direct Connections, and if you want to try online, set the visibility to something other than offline). Alternately, after starting a new game or loading a save, hit Esc, open the Connection Menu and set the options there; once in-game you can also click the diamond icon at the top of the mini-map (in the keyboard UI) to change these settings.
With another instance of the game started on the same LAN, in the main menu click Join, switch to the LAN tab and the host game should be visible (assuming the host system has a game open, in the game itself or character creation).
Divinity Original Sin Enhanced Edition - 4 Player ModFor the extracted main.pak file, the only file you need to worry about is the one you edit. It should be placed in the install folder:
..\Divinity Original Sin Enhanced Edition\Data\Mods\Main\meta.lsx
4 Character Creation For the EEIf you get the mod from nexus, rather than the Steam workshop, you will need to extract the pak file to the '..\Documents\Larian Studios\Divinity Original Sin Enhanced Edition\Mods' folder. See the mod description and readme file for details.