Will bringing him up to rank 4 help you complete harder content? If yes, I'd say rank him up. If not, wait. Speaking from personal experience, you can wait a longgggg time for a specific champion. I'm still waiting on a few.
He is the best champ in the game (IMO) for taking on the Freezer Burn node that has been alternating with Biohazard in the daily Invasion quests this month