The operating system that is running the PlayStation 4 has been revealed to be FreeBSD 9. The operating system named Orbis OS, is essentially the same as the FreeBSD 9.
The preference of FreeBSD over Linux is due to the more liberal licensing that comes with it. Also the PS4 is now x86_64 based rather than being Cell-based which facilitates the seamless use of FreeBSD.
It comes as a bit of a surprise, though, that FreeBSD is being used when the PS4 runs on an AMD eight-core APU with Radeon graphics. Catalyst driver for any BSD operating system is unavailable right now and the open-source driver ported from Linux with Radeon KMS and Gallium3D is still in a primitive state. We can only speculate that Sony and AMD has developed their own graphics driver or at least ported the Catalyst code-base to BSD.