While Sony hasn't yet officially announced the PS4, no doubt it's coming. The PlayStation 3 will get a successor. According to a "trusted source", work has started on PlayStation 4 games.
Website Develop reported that "various game projects are at very early stages". Apparently, Sony's in-house studios are starting development first, which makes obvious sense.
It seems like Sony is just starting on the PS4, and the console appears to be a few years out. We probably won't see anything until 2013, at the earliest. Next year, Sony will most likely focus on the PS Vita.
This past May, a senior Sony executive said work had begun on a future platform—a comment Sony later said referred to the Vita.
According to one report, the next PlayStation will have Kinect-style motion controls.
In Februrary, Sony exec Kaz Hirai indicated that Sony was not yet working on the PS4.
Kotaku is following up with Sony and will update this post should the company comment.