If you are looking at stuff for youtube videos, before worrying about a 4K capable camera body I would look at lenses, microphones, sound recorders, lighting and if you go for the 4k make sure you have a computer capable of editing 4k video and an internet connection that won't have you waiting weeks to upload one video.
So in summary, don't worry about 4k - there are plenty of other things that are more important in video production for YouTube.