Yep, I find it very hard that PS4 will sell less than 120M. I think even Wii with monstrous drop sold 9M after the announcement of WiiU.
I expect between 108M and 120M sold by the time the PS5 releases; based on a March 2020 or Nov 2020 release respectively.
As for sales after the PS5 releases, I expect the droop to be more than it has ever been in playstation history. So say around 15M of extra sales spread out through 2021 to 2023 after which I expect the PS4 t be discontinued, which should bring its Lifetime total to around 123M to 135M.
The reason for this drop will be BC. I believe the PS5 will have BC with the PS4, PS3, PS2 and PS1.
More importantly BC means that current PS4 owners will have very little reason to hold onto their PS4s. That means a lot, because as the rapid uptake of PS5 begins over its first 2 years there will also be a marked increase in used PS4s and PS4pros on the market. These used consoles will no doubt be cheaper than whatever price the PS4 will officially be at the time. We are talking people being able to get a PS4 for as little as $130 and a Pro for as little as $220. That will no doubt affect sales of brand new hardware.
I also expect the PS5 initial uptake (first 3 years) to be even more aggressive than that of the PS4. Even though I expect the PS5 will come at a launch price of $499.