Sure, PS2 sold more hardware, but PS2 also had less/weaker competition at the time as well.
PS4 sold software like it was free, while competing against what was, or should have been, much stronger competition.
I'd say their success is pretty much equal for one main reason. While it was likely both would be a success in general, nobody anticipated how swiftly either of them would absolutely crush it, prior to their initial unveilings and specs being officially presented.
PS1 - ! - We must build a console that can alert our enemies.
PS2 - @- We must build a console that offers online living room gaming.
PS3 - #- We must build a console that’s powerful, social, costs and does everything.
PS4 - $- We must build a console that’s affordable, charges for services, and pumps out exclusives.
PRO -%-We must build a console that's VR ready, checkerboard upscales, and sells but a fraction of the money printer.
PS5 - ^ -We must build a console that’s a generational cross product, with RT lighting, and price hiking.
PRO -&- We must build a console that Super Res upscales and continues the cost increases.
