110m is very likely. Perhaps 115m.
In case PS5 hit the market in 2020 or before, I find hard to believe that it can hit 120m. Unless PS5 has a hard time and they make PS4 very cheap and people keep buying PS4s while PS5 gets hot. However, if PS5 gets hot and considering how PS4 fans are happy with Sony these days (I am), the jump to PS5 will be huge. In that case, I'd say that PS4 can go from 100m by the end of 2019 to 110m by 2021.
So, I'll vote 111-120, but I believe it's unlikely to hit 120 if PS5 is a success. Perhaps, it can hit more than 120m after many years, in 2023, but I wouldn't dare predicting 2023 numbers today.
God bless You.
My Total Sales prediction for PS4 by the end of 2021: 110m+
When PS4 will hit 100m consoles sold: Before Christmas 2019
There were three ravens sat on a tree / They were as blacke as they might be / The one of them said to his mate, Where shall we our breakfast take?