After twisting the knife at E3 regarding Microsoft's then-complicated game-sharing program, Sony has explained how digital strategy will work on PlayStation 4. Although sharing physical games remains as simple as Sony's tongue-in-cheek video suggested, games bought over PSN will be more locked down. The first PS4 you register will be your "primary console," and that's where digital purchases will automatically sync to. Anyone logged in on your primary system will then be able to play that game.
If you download the game on a secondary console, only you will be able to play that game. There's no limit to the number of consoles you can download a game on, but only two games can be played concurrently — one on your primary system, one on a secondary console, which you must be logged into. Sony has also revealed you'll be able to change which PS4 is your primary system if you need to. It's a slightly complex system, but it's not too different to how digital purchases work on PlayStation 3, and offers a little flexibility for gamers that usually download rather than buy physical discs.
There are a few other details over at the PlayStation 4 FAQ on Sony's site, so if you're picking up a PS4 at launch you can head over to read up on user accounts, friends lists, and all the rest.