Initial mod to allow more than 4 followers. I've done initial testing, playing as Fane and adding all the other origin characters in Fort Joy. I've successfully tested out combat, bartering, companion attitude change, companion quest lines in Act 1 (including one companion being removed and added back into the party), formations, character sheet and inventory menus. I have confirmed that companions transfer over successfully into Act 2.
Note: On existing saves the mod works best without a full party / with no-one yet recruited (or start a new game). However Zlayer has confirmed that dismissing and re-recruiting companions will allow the party size limit to be removed.
Character sheet menu is a little wonky if you use the arrows to navigate. It's still completely usable, just the behavior is weird. You can also very easily just click on the portraits in the sidebar to switch between characters.
Barter menu has party members going across the top of the screen but you can still click on all of them
Whenever all companions want to comment on something (e.g. everyone has an exclamation mark above their head), only 3 of the 5 will comment (the first three you recruited). I am in the process of trying to sort this out but it might not be feasible. It seems the reflection and party decision dialogue scripts are tied to co-op functionality and so trying to overwrite the 4 person limit here may not work as well.
Some conversations treat you like you are alone if you have >3 companions such as Margo and some of Gawin's dialogue (although Gawin still retains one choice that acts like you have companions with you). These are not using the party full flag and it may be an overflow error on those specific dialogue choices that I need to manually edit individually. I will look to see if I can push fixes for these later, but for now dialogue editing is not enabled in the Divinity Engine. They aren't super gamebreaking (you can normally choose the alone options even if you do have a party, it's just that sometimes the group option is gone). Note that other more important story instances that do reference your companions are intact like talking to your god during the Braccus Rex vault statue or statue near the pigs sequence