The PS3 sold ~500k units in Japan after the PS4 had launched. The PS4 has flatlined in the last several months, so it would already be an accomplishment if it managed another 200k until the end of its life.
The PS3 sold ~1.6m in the USA after the PS4 had launched. The PS4 hasn't been in any better shape lately, so that's another major market where it will fall short. October NPD painted a worse picture than it actually is, but it's easy to recognize in which direction the wind is blowing nonetheless.
Globally, the PS3 sold ~7.5m units after the PS4 had launched. The PS4 will have to rely on Europe (mostly the eastern half) and mainland Asia to match or exceed that figure.
However, the actual target is ~11.4m achieved by the 3DS, so for the PS4 Europe and mainland Asia will have to pull a lot of the weight. Which means that Sony's strategy for those regions will have to drastically differ from what they are doing in Japan and America.
Alternatively, we can calculate with shipments. The first thing to know is that the PS4 has sold through ~114.0m by the time the PS5 launched, so it needs to get above 125.0m in its lifetime. The decline in shipments has accelerated: In the fiscal year ending March 2018 the PS4 had shipped 19.0m, followed by 17.8m and 13.6m; the current fiscal year is projected to be 9.0m which would bring the PS4's LTD shipments to 119.4m. In other words, another ~6.0m will still be missing by March 2021 and that's a longshot when you consider that Sony wants a fast transition from the PS4 to the PS5.
For too long Sony has not made one last big push to increase PS4 sales (i.e. a price cut) and they aren't going to change their mind now when the PS5 is what they want to sell. The PS4 Pro's production has likely ended already, because its price point makes it pretty pointless in comparison to the PS5 SKUs. A standard PS4 for $/€300 doesn't look great when people can buy a backwards compatible $/€400-500 next generation console. Plus there's the used market which is going to have an increased influx of second hand PS4s due to not-so-well-off people trying to get the funds for a PS5 and a bunch of games.