Sounds to me like Jenova DEATH. Its palette swapped with LIFE and BIRTH, and is also 'changed' into Sephiroth's form, making this the literal reason they look alike.
I can't think of a sensible reason for '4th' though, except perhaps the fourth continent, but LIFE is there too so thats not much help. I'm really only guessing DEATH over the other two because the word is in the clue.
If I'm right someone else can post... though I feel bad if it is as this really isn't solved.
