Earlier this month, YouTube ran an experiment asking some free users to upgrade to a premium account to watch videos in 4K resolution.
The company appears to have stopped this test – likely due to the disruption it caused users.

In a tweet, YouTube confirmed the end of the experiment and said that, "viewers should now be able to access 4K quality resolutions without a Premium subscription." So all users can now watch videos in the highest resolution without paying any subscription.
Google charges $9,99 per month for YouTube Music and $11,99 per month for YouTube premium in the US. — these prices vary around the world. The company claims to have more than 50 million paid subscribers to both offerings worldwide. The Premium subscription offers features such as ad-free video viewing, video downloads for offline use, and background playback.
YouTube is testing many ways to convert free users to paid users. Earlier this year, he ran an experiment showing users up to 11 ads without the ability to skip before the start of a long video. This test was stopped within a few days.
