Posted as of 02/21/13, PS4 will play used games:
http://www.eurogamer.net/articles/2013-02-21-the-big-interview-sonys-shuhei-yoshida-on-ps4
[quote name='Shuhei Yoshida, Eurogamer']
Eurogamer: One of the questions my readers really want an answer to is whether you're going to block the use of second-hand or 'used' games, because it's a huge concern for them.
Shuhei Yoshida: Do you want us to do that?
Eurogamer: No. I think if you buy something on a disc you have a kind of moral contract with the person you've bought it from that you retain some of that value and you can pass it on. Do you agree?
Shuhei Yoshida: Yes. That's the general expectation by consumers. They purchase physical form, they want to use it everywhere, right? So that's my expectation.
Eurogamer: So if someone buys a PlayStation 4 game, you're not going to stop them reselling it?
Shuhei Yoshida Aaaah. [Asks PR adviser.] So what was our official answer to our internal question? [Consults adviser.] So, used games can play on PS4. How is that?
Eurogamer: That's great. So in past generations, you had the PS2 with third-party exclusives, amazing marketing and the DVD player that helped it to be successful. With the PS3 the exclusive games were harder because it cost too much and the marketing was harder because at that point everyone else was making cool-looking electronic devices too, but you still had Blu-ray to give you an advantage. [/QUOTE]