Based on the PS3 sales curve, 125M. Both systems show very similar curve, with PS4 having stronger first 3 years (vs its peak), but it also dropped by around 5% in last year or so, compared to PS3. So if we take PS3 sales curve from years 6 to 9, adjust it by -5% and plug PS4 numbers, we'll end up with around 125M by the end of 2022. Of course that's if PS5 releases in 2020 as it'd correlate with the PS4's time of release in PS3's lifetime.
This is not 100% accurate because PS4 already surpassed PS3 by a mile, I think PS4, especially PS4 Pro, will be more enticing for gamers after PS5 launch than PS3 was after PS4, the PS4 has mooooore room for price cuts than PS3.
I think they will release PS4 Super Slim close before PS5.