It has been officially confirmed that the PlayStation 4 will not support PlayStation 3 games natively. This means that any retail (i.e. disc-based) or digitally downloaded games from the Playstation Network will not be playable on the new console due to the x86 architecture.
In a roundtable with journalists after the PlayStation 4 reveal, Sony’s Shuhei Yoshida, President of Sony Worldwide Studios, elaborated further on the question of backwards compatibility.
In the IGN article linked above states:
“In a roundtable conversation with journalists, President of Sony’s Worldwide Studios noted that PS3 games of both the retail and downloadable variety will not be compatible with PlayStation 4, at least not natively. In other words, your PS3 discs won’t work in the new console nor will the games you downloaded.
There’s a catch, though: emulation. The PlayStation 3 doesn’t read PlayStation 2 discs (at least not anymore), but it can play PS2 games via the PlayStation Store in an emulated environment. Yoshida concedes that this is possible for PlayStation 4 supporting legacy PlayStation consoles as well. Indeed, the Gaikai presentation during PlayStation 4’s reveal touched on this “everything, everywhere” mentality. But for now, downloaded PS3 games won’t work “unless, somehow, some games work on emulation. And the easiest thing, technically, would be to make PSone games work on PS4 with emulation. But we’re not talking about our emulation plans as yet.”
In short, “There are two ways [to play legacy content]: emulation or cloud services [from Gaikai]. But native support [of digital games from PS3], no, sorry. It doesn’t work.”
Gaikai and Cloud Streaming
Based off of statements made at Sony’s PS4 reveal event, it is believed that Sony will be using Gaikai’s capabilities to provide access to games from previous PlayStation consoles through Gaikai’s cloud streaming capabilities.
At this point, it is not known whether this will require players to re-purchase games they already own, although this seems quite likely.
Visit IGN’s PS4 Gaikai Integration wiki to learn more.
Sony has confirmed that users across the PlayStation Network will be able to communicate back and forth on the PlayStation 4 and PlayStation 3. The catch is that only text-based communications will be delivered across the platforms. Any and all voice or video messages will go undelivered between the two consoles.
Official Source: http://www.ign.com/wikis/playstation-4/Backwards_Compatibility