Sony has clarified PS4 game sharing.
The first PS4 system you register will be your “primary console,” and that’s where digital purchases will automatically be synced to. Anyone that is logged in on your primary system will then be able to play that game.
If you wish to play PS3 games on the PS4, I believe their is a fee of like around $9 for 2 or 3 ps3 games, but that may change anytime. Sony can change rules on anything they want.