Those USA numbers are indeed NPD, but i'm telling you those spanish numbers are not sell-through i saw the same GfK data in Spain saying that, so i can't believe any of the other european markets that are probably using the same tracking system. And in the end, the same guy put the total of PS4 and Switch using shipment numbers, so there's something fishy in this data.
This article from Vandal, the very same page that give us updated numbers from software in Spain put this article.
It shows the numbers from 2015 through 2018 and they are what i posted earlier. Between 500-600k each year. And i have to remind you that PS4 had an excellent start during its first year too (ending 2014).
Then this other article citing Sony itself saying PS4 have sold 3'3M lifetime around mid-half 2019 since launch.
That graph you brought says PS4 had already sold 3'6M by the end of 2018. If Spanish numbers are wrong, i can't trust any of the other european numbers showed in there.
Your numbers arent adding up either. The 2015-2018 numbers you posted add up to 2.27 million and they reported 134k for the 2013 launch putting it to ~2.4 million
The 3.3 million figure is from end of March 2019 so that would mean 2014+Q1 2019 would be about 900k which would make 2014 the peak year anyway.
My personal theory is that the Spain numbers in the link I posted also include Portugal, similar to how Northern Europe & Benelux were blocked together.
When the herd loses its way, the shepard must kill the bull that leads them astray.