So Sony are not making any fanfare regarding the 20m milestone, so probably more 1st party games will quitely pass the threshold if they haven't already
They will potentially have 4-6, 20m+ selling exclusives by the time PS4 is discontinued. They're selling 1st party titles at near Nintendo levels
Hmmm I'm not too sure about this. I think that 4 is the maximum number of 20M sellers that the PS4 will get for exclusives. My guess for the next two to three will be God of War, Uncharted 4 (probably has already hit 20M, otherwise it likely wont), and TLoU2.
Horizon: Zero Dawn reached 10M just last year. Unless there is a BIG boost right before Horizon Forbidden West comes out, it'll probably linger around 12-15M. Gran Turismo Sport hit 8M last year and, with a new one launching very soon, it's unlikely that it has legs. 10-11M total for that one is my guess. The only one that I am not sure about is Ghost of Tsushima.
Something Sony can do to increase sales of their titles for PS4 is to create PS5 branded PS4 games, similar to what Microsoft did with select Xbox 360 games (where they essentially put it in an Xbox One box, and said it was for both consoles). If they do this then I think that one or two more games may become 20M sellers.