I only watch YouTube on my main PC and laptop. Neither of which are 4K and I don’t own a 4K TV.  However, many people like to watch YouTube in 4K (especially on their mobile devices) and have been doing so for free. That is likely changing… You might soon need YouTube Premium for more than ditching ads, background playback and trying new features. As MacRumors notes, users on Reddit and Twitter have noticed that YouTube is asking some viewers to upgrade to Premium to watch videos in 4K resolution. It’s not clear which countries, devices or videos are affected, but this won’t be thrilling if you…