4K is the stage after full (1080p) HD and the video standard that will be best experienced on those new Ultra HD television sets. The thought that a smartphone brings that level of processing power inside it is quite impressive.
The following video sample captured by the Liquid S2 is at a 3840 by 2160 pixel resolution and you will have to select 'Original' from the playback options in order to see the video displayed as it should be.
Keep in mind that the review unit used wasn't yet finalized so future software optimizations may change how the smartphone's 4K video looks.
You can also check out some photo samples captured by the Acer Liquid S2 camera.