For those hoping they'll be able to use the PS4 instead of the server for 4K playback, it doesn't look like that will be possible (at least not at launch- maybe Sony will add it as a firmware update like they added 3D to the PS3).
See this PS4 FAQ:
What screen resolutions will PS4 support?
PS4 will support 480p, 720p, 1080i, and 1080p video standards via HDMI output.
Will PS4 support 4K content for video or gaming?
Support for high-resolution 4K output for still images and movie content is in consideration, but there are no further details to share at this time. PS4 does not currently support 4K output for games.