Yes, both the iPhone 6s and the iPhone 6s Plus capture 4K video. 4K video resolution is 3840 x 2160 pixels at 30 frames per second (fps).
The native Camera app is also capable of taking 8-megapixel still photos while simultaneously recording 4K video. The iPhone 6s Plus features Optical Image Stabilization (OIS) while recording 4K video as well.
Additional features include video geotagging and playback zoom. With playback zoom, users can pinch to zoom while a 4K video is playing on the iPhone 6s or iPhone 6s Plus to see more detail.