The Operating System is called 'Orbis OS'. It is a modified version of FreeBSD 9.0.

We arent sure if this will bring again the 'Other OS' functions to Playstation 4 (remember that this option enabled Playstation 3 to install other OS in the console like Linux or Windows).

VGLeaks has posted some 'screenshots' of the boot-up of an second-generation devkit of the PS4 similar to the ones used for game demos at E3, and it seems the OS uses an form of 'FreeBSD' and opens up rumors if it might be possible to install Linux on the PS4, or if Sony might be planning again to allow it as an option in the future.

When you boot up a second-gen development kit, you will be prompted with several options, as you can see in the next captures:

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More options appear when you choose startup settings:

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As you can imagine you have either a console mode or a graphic mode (the one you will see in a retail Playstation 4).
Here you can see some images of the console mode and some of the files/directories in the devkit:

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We'll try to add more information in the future.
(Click the above picture thumbnails, for close-up views)

NEWS SOURCE: Details about Playstation 4 OS development (via) VGLeaks