4K Video on the Nexus 5
The sensor on the Nexus 5 is 3264×2448 pixels and the Camera2 API on Android Lollipop allows video capture at full resolution.
There are other apps which support capture at this resolution but this has an aspect ratio of 4×3 and needs post processing for display on a UHD TV.
Snap Camera is the only app that supports UHD video recording with a 16×9 aspect ratio.
Two resolutions are supported, 3840×2160 (4K) and 3264×1836 (3.4K). 3.4K is the native resolution while 4K is interpolated. Both resolutions will record at around 30fps in good light.
The default bitrate is 48Mbps but this can be changed. The Nexus 5 supports up to about 150Mbps.