So besides mobile consoles, PS2 is the indisputable king of home consoles from gen to next gen.
There is something amazing too from those numbers. PS3 almost stop selling after PS4 was out.
But PS1 sold a lot more reaching more than 100 million units.
Acording to that info PS3 sold like 7 Million more after PS4, but PS1 sold 30 Million more!
And with 94 Million units shipped, PS4 can sell very close if not surpassing PS2 from gen to gen.
A big reason for PS1 & PS2 having such long legs is that they released much later in many developing countries. This isnt nearly as true for PS4 so its legs likely wont be nearly as long.
It will be better than PS3 but much less than PS1, I'm going to say 15-20 million.
When the herd loses its way, the shepard must kill the bull that leads them astray.