The PS4 is not backwards compatible with PS3 discs. Sony plans to allow you to STREAM PS3 titles from the Gaikai service to your PS4, kinda like how you can stream a game from the PS4/PS3 to your vita.
The downside is lower video quality, increased input lag from the controls and the requirement to have a sufficient internet connection. While its better than nothing, it certainly is not nearly as good as being able to play the game right from the console itself, be it disc or a PSN download. You can basically forget about competitive online play, or any game that requires quick reaction/timing.
I hope you are joking by thinking tablets will replace consoles. If anything, you should compare tablet gaming to other mobile devices, like smartphones, DS and Vita systems. If you think playing Facebook games on your tablet are going to be better than playing full high budget games on a dedicated console you are sorely mistaken.