To better understand things, it works this way... You can post what ever quality you wish on YouTube from 360 to 4K. However, YouTube wants to play back content as quickly as possible for a viewer, therefore it is YouTube who adjust the quality of play back to match the viewers download capability. That means that a 4K video streaming to someone with extremely slow internet, will be reduced to what ever the minimum quality can be in order to get the content to the viewer the fastest way.
So a slow speed internet will get video content to the viewer quickly but that 4K will be reduced down to possibly 320KB. At the same time someone with extremely fast internet service watching that same 4K video will be able to see that video just a quickly but it will stream in 4K because the speed is there to allow that video to go through.
YouTube do not like to make customers wait for content to be streamed, so they reduce the quality to get crap there quickly and allow high end stuff to get there when the internet speed of the viewer allows for it. Conversely, Vimeo will always play video content at the quality it was uploaded to their site. Unfortunately this means that someone with a very slow internet speed will have to wait ages for the entire content to download, before they can view it, or it will be shown in spurts as the content arrives. Now if another person streams that same 4K video with fast internet at the same point in time, they get to see it instantly on Vimeo, whereby the slow speed user has to wait, though at least they both receive a nice 4K quality.