It was only a couple years ago that shooting HD video from our phones was a novelty. Now they're capable of cranking out four times the resolution of most consumer HD TV's.
Samsung took a major step forward with the camera they fitted onto the Galaxy S5. Not only is it a higher resolution sensor than the one found on the S4, but it's also a physically larger sensor, allowing for a more photographic or cinematic depth of field (that pleasant background blur behind the subjects of your pics and vids).
Shooting 4K can be fairly taxing. Thirty times a second, the camera is having to process a high quality 3840x2160 image (that's 8 megapixels) synced with high quality audio. This means that Samsung has to deactivate other software processes like electronic image stabilization. File sizes are also pretty enormous. Twenty seconds of 4K video clocks in at around 130MB, and the camera is only capable of buffering about five minutes of video at that size and bit rate.
Still, the combo of larger sensor and UHD resolution can produce some stunning results. Hit the related video to see our full real world benchmarks of the GS5 camera shooting 4K video in bright outdoor scenes, low light, tracking movement, and utilizing the zoom!