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.
So what or where do you expect the PS4 to be at the launch of the PS5?
If PS5 releases in March 2020: PS4 2019 (15M) + PS4 2020 (5M) = 111M LTD PS4 sales. In this case I could say its lifetime sales would be hardly any more than 120M
If PS5 releases in Nov 2020: PS4 2019 (15M) + PS4 2020 (12M) = 118M LTD PS4 sales. In this case I can see lifetime sales being around 130M