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You just need to have 4 characters of the same path on the final boss fight, so you can use other characters leading up to it.

A reference to the 2013 sci-fi thriller movie "Coherence." In the movie, a group of friends had to fight off their alternate selves and regroup with their friends from the same reality, similar to how this achievement requires 4 characters from the same path. - Credit: Necrosyrtes