To test out the Sony Xperia Z2's 4k recording, Cam took his phone out across to the front of his house and tested a few of its basic functionalities, including testing the auto-focus and auto-exposure speed. In all, it's pretty good. It does create some noise once the light levels drop, but that's to be expected from a camera sensor as small as this. 

There was a small delay in response from the camera when in came to focussing and adjusting the exposure, but it was pretty smooth and didn't do a lot of "searching". Once it had a target, it locked on. Some times, I found that it would focus on the background instead of on the foreground, and there's no way of manually picking a spot once you're started filming. Other than that, noise level got higher as light dimmed - as you'd expect - but it's a pretty decent camera. 

Sony Xperia Z2 unboxing - https://www.youtube.com/watch'v=jBL5h3yNzq8