2 > 1 > 4 > 3
Is how it will most likely end up. PS3 still has a ton of gems but I expect PS4 to outdo it looking at its upcoming lineup and potential.
This is what I think too. I think the PS4 library has already surpassed PS3 or will by the end of its lifespan. A lot of the strong Japanese console game development that faded during the PS3 years has come back on the PS4. It's not as strong as it was on the PS1 and PS2, but it's recovering.
I don't think the conditions will ever again exist for a single piece of dedicated console hardware to have more games developed for it than the PS2, or even the PS1. They each had thousands of retail games released for them, almost 4000 in the case of the PS2 if I'm not mistaken. The industry was just different back then.
Dedicated consoles, and really PlayStation, took up a much bigger chunk of the overall game market back then. Today a lot of that air has been taken up by mobile and PC, and console games are more expensive to make. Indie games are filling in the gaps on PS4 but probably won't reach the PS2's numbers. I don't think a console is ever gonna have a library like that on it again unless it either pulls from like three hardware generations of backwards compatibility, or the console manufacturer is far more open with who can develop on the system.
This is why people keep getting so pissed about PS2 BC on the PS4 -- the PS2 probably has the most valuable library of any game console. Not only is it absolutely immense, but it represents an era of refinement in 3D game design, containing a huge number of what are still some of the greatest games of all time. There is no way Sony or other publishers will ever be able to cover enough of the PS2 library through remasters and individual re-releases. Nothing short of full emulation with disc compatibility will even begin to cover it.
PS2 > PS1 > PS3 > PS4
Being a PS4 only owner this gen is like I imagine being a Nintendo only owner since the N64 has been; long dry spells with no games I would even bat an eyelid at.
Hell. The Fuck. No.
I was a Nintendo-only owner during N64 and Gamecube. There is NOTHING as bad as that was outside Nintendo consoles. Whole damn genres were basically missing from those libraries. The closest things to an RPG on the N64 for almost its whole lifespan were Quest 64 and something called Aydin Chronicles. Other than Smash, basically no arcade 2D fighting games between the SNES version of Street Fighter Alpha 2 in 1996 to the Gamecube version of CvS2 in 2003. Those dry spells made me buy and enjoy C and D-tier games. Only upside is that the N64 had kind of a lot of FPSs on it. Gamecube was a bit better but still pretty barren. Still only a handful of RPGs, massive gaps in the third party support everybody else got, and often the most inferior versions of multiplat games not because of the hardware, but because the devs just didn't give a shit. The Gamecube got like one game that even resembled a racing simulator: R-Racing Evolution, that game that was pretty much advertised based on some CG J-Pop model's tits.
Maybe my view of the PS4 is a bit skewed because I also have a gaming PC now, but if I only owned a PS4 I imagine I'd at least have shit to play. I wouldn't be dealing with massive gaps in third party support. There are actually RPGs and fighting games and racing games to play on the PS4.