It feels like Unity is catching up to Unreal 4 finally in the graphical department. If Unity does catch up to Unreal 4 graphically and stays free, it is going to steal some AAA marketshare from Unreal 4 for sure, since Unreal 4 costs 10% royalties of a game's sales.
I can see Unreal 5 if Unity gets too close. Really, I've seen more from Unity that's prepping for Gen 9 than Unreal. So, I'd bet on it.