It's ridiculous really. Apple launches a 5k retina iMac and the iPhone 6 still doesn't support 4k video recording.
I ended up getting a Panasonic LX100 to record 4k video, but before that I had a Note 3 and the 4k videos on either are absolutely fantastic on the 5k display of the iMac.
I suppose 4k will come when the iPhone finally goes above 8megapix in resolution, but by that time it will be an incredibly meh feeling.
Hopefully Apple somehow manages to pull a 5k video recording mode with h.265 / HEVC hardware encoding in the 6S. But probably not.