There’s an assumption that when a newer, more powerful generation of hardware comes out, it will possess the capacity to play games from the previous generation. Blame Sony, as the PlayStation 2 was the first mainstream home console, since the gaming crash in 1985, to do so. The initial run of the PlayStation 3 also had a hardware solution for PlayStation 2 emulation, but later versions of the console dropped this for a software method (akin to what the Xbox 360 uses to play some original Xbox games). Eventually, however, this was dropped entirely. Now, the occasional PlayStation 2 Classic makes its way onto the PSN, while more popular PS2 games get HD rereleases.
Clever Beans, a developer based in the UK, has now stated that the PlayStation 4 will most likely not be backwards compatible with the current generation of hardware. This is due to presumed difficulties with emulation given that the PlayStation 4 is expected to eschew the PS3’s Cell architecture in favor of an AMD x64 CPU and AMD Southern Islands GPU.
On the other hand, streaming is a possibility, and would provide some context for Sony’s acquisition of Gaikai earlier this year. But PlayStation 3 games are notoriously massive, and streaming would require an extremely high-speed connection. This is largely based on unconfirmed suspicion, though, and should all be taken with a grain of salt.
Source: PlayStation Universe