According to a new report from the Wall Street Journal, the PlayStation 4 will be able to stream PlayStation 3 games using Gakai's technology.
Quote from the article:
"The new technology, to be unveiled Wednesday along with the new console, will allow users to play games delivered over the Internet, these people said. The streaming service, they added, is designed to use current PlayStation 3 titles on the new console; the new device is also expected to play new games stored on optical discs"