Have you noticed that when you get a new iPhone, all the videos you record on it are not 4K resolution. There is nothing wrong with your phone. Default setting on iPhone is to record videos at 1080p@30fps. I guess reason for that mid level setting is to keep your space usage under control. As video resolution increase, size of video file increase as well. If you want to turn on 4K recording on your iPhone6 series, follow these steps.
In your Settings > Photos & Camera > Record Video setting, change the option to 4K@30fps.
You can view these steps in our video as well.