About 4k footage

YouTube compresses several versions of all videos that you upload. They automatically choose the frame size that will stream in a reasonable time according to your available bandwidth. 4K videos seldom are automatically sent out at 4K. 720P is a reasonable expectation for an average bandwidth connection. You can choose to download the video at a higher frame size, but you will have to wait for it to download unless you have a fast connection. Here is where you find the option to playback at a higher frame size:

There is one more thing you should know about YouTube. You will get the best result if you follow their format guidelines and compression settings. If you do not follow their recommendations, their software will recompress your video with a sledgehammer, and the video quality will suffer. If you want the best results, you need to use the YouTube presets in the Adobe Media Encoder. They match the YouTube standards and will give you the best results.