So my holy priest has reached 90 after 4 rebirths.
She has taken all of the holy talents.
She has taken all of the balance talents, except Strength of Will, in which she has none.
If she rebirths a 5th time, she gets 5 more talents, which she could put into Strength of Will, but only 5/20.
So there is no ultimate to be gained.
She will absolutely not put any points into shadow, because all ultimates there are shadow related, and since she has never put any points into shadow affinity, any shadow spell would be very weak anyway.
My question is: What purpose would a 5th rebirth on holy priest have, other than the shiny circle around my cute head?
You get 5 talent points of improved armor and magic resistance. This will make you more durable, more of a battle priest-tank.
I felt the same way after my 4th rebirth, but I soon became bored running circles in Zorian (see my other posts about level 90), and then decided to rebirth anyway. The level 90 fully maxed out holy priest with legendary full gear is pretty awesome to play. You feel quite indestructible at times.
But, you could also leave it at your 4th rebirth too. Run Zorian for a month, you'll be itching for something else to do, and those talent points will be waiting for you.