In terms of BC, retro game catalog support, grandfathering purchases, cross-buy, cross-save, upscaling, and the like, they're probably the leader in this arena.
If they actually announce disc based BC for PS2 on PS4, they'd be the irrefutable king of it, but I sincerely doubt they will do this because of publisher relationships and licensing.
But why? I mean, the PS2 was able to play PS1 games - Sony very probably didn't look forward that much and made license contracts with each publisher to have them allow PS1 games run on PS2. Same with PS2 to PS3.
I think those much-cited license issues are only a problem with digitally distributed games. Sony are not damaging any property or license when they allow discs being played back. If that was the case, any emulation (on PC or whatever) would already have been sued to the moon.