Doesn't PS Now let you stream PS3 games to PS4? If that's possible I don't see why they can't put PS3 games up on the store to play on PS4.
If you mean putting Yakuza 5 up on a Now type streaming service, they could and should do that. If you mean running Y5 natively on a PS4, that's not remotely the same.
Now can stream PS3 games to PS4 because it's basically running the games on a PS3 and outputting the video over the internet. Running them natively is next to impossible currently as the PS3 is a really unique piece of hardware. Emulating it would be a major task, and might even be impossible on the PS4 hardware. To get Y5 on PS4 natively, you'd basically have to remake the game.