I think that more than the 4th mugo (I'll be calling him Ancient to make it easier) being the mix of the other 3, he holds the memories that connects all of them to the original. What I mean is that for some reason (trauma, spell, whatevs) Mugo's personality shattered into 4: the childish Mugo (Monster), the horny (teenager) impulsive Mugo, the mature but tired Mugo (Old Man) and the one who holds all the memories suppressed so they can continue living without despair and was waiting for his lover to return (Ancient).
I think that more than the 4th mugo (I'll be calling him Ancient to make it easier) being the mix of the other 3, he holds the memories that connects all of them to the original. What I mean is that for some reason (trauma, spell, whatevs) Mugo's personality shattered into 4: the childish Mugo (Monster), the horny (teenager) impulsive Mugo, the mature but tired Mugo (Old Man) and the one who holds all the memories suppressed so they can continue living without despair and was waiting for his lover to return (Ancient).