I wouldn't be surprised if the 4 member limit is mostly due to the "Toilet Chain" which is either someone's pride and joy or so embedded into the Divinity Engine as to be unfixable. How else to explain Larian's absolute refusal to improve it.
On console I have found that the " Adjustable Party Limit" Mod (especially when combined with Tactician Enhanced) to be excellent, and well designed. However they still aren't able to solve the many issues created by the Toilet Chain and in fact make them worse.
