It has been reported by Tuaw.com that the developers of the WALTR video upload app discovered this great news whilst testing their app. At the moment you are not able to transfer 4K files using iTunes. If you want to transfer this kind of file then you will have to use an alternative service to get transfer them to the new model iPhones.
The quality of the media will be limited by the screen resolution, so I guess the larger and higher screen resolution iPhone 6 Plus is a better device for viewing 4K videos than the iPhone 6.
Although the iPhone 6 can’t shoot 4K video, it is a pretty cool feature. Because of this and wild rumours that the iPhone 7 will be getting a camera boost, (it may allow shooting in 4K video), Apple appear to not be touting this is a main feature of their flagship device.
With the new iMac Retina 5K display,  it is pretty clear that Apple is looking to higher resolutions in their products.