You can play your PlayStation 4 games on any PS4 console, anyone can play games and use your PlayStation Plus features on your primary PS4, and your PlayStation Network account can be used on two systems at the same time, according to Sony. Those and other FAQs were answered this weekend and announced via a tweet by Sony Computer Entertainment President Shuhei Yoshida.
The sharing model makes sense, as it gives you full freedom to enjoy your games on whatever PS4 system you want, and there are safeguards in place to prevent account sharing with other players. For example, you can download a game you bought to any PS4 system, but only you can start a game on the console using your account. Of course, you can sign in to your account for a friend if you're in the room, and you can give your friend your credentials, but the two-system limit prevents gross abuse. It's in the spirit of sharing without piracy.
Your primary PS4 system can be changed, but only after deactivating your current primary system. No matter where you download a game from, it will be automatically delivered to your primary PS4. If you own a second-screen PlayStation Vita, it syncs with just the primary system.
The device sharing model used by PS4 is similar to the one Xbox 360 uses, but a little better. Xbox lets you download use multiple systems and app installations, but you can't use your account on two systems simultaneously with Xbox 360. An argument can be made that this makes sense as well though, and multiple concurrent system log-ins are not an allowance Microsoft is willing to make as yet.
The other issue addressed in the new PS4 FAQ is friend lists. You can have up to 2,000 friends connected to your PSN account. All 2,000 are viewable on the PS4 as well as on the PlayStation Vita device and the PS app, but only the first 100 friends are viewable on older PS3s.