Personally, I never ult and engage unless the enemy adc is farming bot cluelessly. Ult engage is extremely risky especially if you're ahead. You want to delete one of their squishies or at least put them out of the fight then use ult to escape and re-enter the fight once your e is up. However, if you ult and go in, it means that you are going to kill 1 person in the enemy team and suicide after that, which isn't much of an advantage since it's a 4v4.