If you upload at 4K they will be available to view at 4K. Whether or not you view them at that resolution is up to a few factors such as internet speed and the selected resolution of the viewer. Stats for nerds shows the 'Current and Optimal' resolutions, so if your settings in the youtube viewer are set to 'Auto' then it will select the highest resolution available based on your internet speed. If you have a fast speed the highest will always be indicated.
However if you have a slow connection, you could click on a 4K video but only see it at 720. If you click on the little cog wheel (settings) you'll see all available resolutions and you can manually pick which one you want. Also, that figure is simply the resolution delivered not displayed. For instance, you can watch a 4K video with a monitor that is only 1080 and it will look great but it's not at 4K but it does have the highest resolution possible.