Currently, buying a digital PS3 game on PlayStation Network lets you access it on up to two activated systems. However, digital PS4 games can be played on any PS4, so long as you're signed in.
A new video from PlayStation explains that any content associated with your PlayStation Network ID will be accessible from any PS4 you log into. "You can access your games and other purchases from these machines so long as you're signed in," the video explains. "Access ends if you sign out."
Of course, if you don't want to be online all the time to access content, you'll be "nominate one PS4 as your home machine." By doing so, "your content is always available on that device, even to other users." Even games downloaded via PlayStation Plus will be accessible to other users on that device.
To sum up, PS4's digital policy is both more and less restrictive than Sony's established system. You can have more "home" machines on PS3 and Vita, but with PS4, you're not restricted to playing your games only on activated systems.