Congratulations: You just found yet another flaw in the 4-man party setup.

In a 4-man setup, 2vs2 ties can happen and there's no fair way to make a resolution.

In a 5-man setup, the 5th person acts as a tiebreaker - It's always either 3vs2 or 4vs1, a clear majority and a clear minority.

Just write it on the:"Why it was a bad idea to make parties be 4 man" list.