Something like 12-15M in 2019, 6-9M in 2020 (or similar) and then the end? Because that's the only way PS4 only does 110-120M lifetime...
It depends on Next gen of course.
Consider PS2 it was at like ~112m at PS3 launch. 70% of its sales. The PS2 had great legs and late international launches like in Brazil.
Consider PS3 it was at like ~80m at PS3 launch. 90% of its sales. The PS3 had poor legs.
Now PS4 is at 91.6m. if we assume that is 75% of its sales then it will go on to sell ~120m. 80% would bring it lower, but I went higher due to PS5 not launching yet.
As I said I think 110m-120m is the safe bet. 105m-115m or 115m-125m would be pretty safe too, but a bit more risky. 100m-110m or 120m-130m would be even riskier, and so on and so forth.