17.8 to 13.6.
That's 4.2 million.
That's a pretty steep drop. About 23.5%
If that same drop holds for the next 12 months, that would mean the PS4 would ship 10.4 million units from April 2020 to March 2021, bringing its lifetime shipment totals to 120.8 million units. That's a drop of 3.2 million units YoY.
But because of the impending launch of the PS5, that drop will be significantly higher. I think the next drop will be roughly 30-33%. That would put the next FY shipment somewhere between 9.1-9.5 million. Just under 120 million.
So, 120 million is the floor. But I think 130 million is the ceiling. I'm guessing somewhere between 125-127.5 million will be the final mark.

Bonus Question: I don't have PS+ subscription and I don't play online multiplayer as a result, so I don't care.

Pancho A. Ovies

Nintendo Switch in Japan (Famitsu): 2018 vs. 2019

PlayStation 4/Xbox One/Nintendo Switch: 2018 vs. 2019