Where can one find a current and definitive reference for video quality as it pertains to hosting HTML5 Videos?
What could do is show that Youtube transcodes to 4K at bitrates of ~20 mbps. You can use a tool like youtube-dl to check. Run youtube-dl -F URL So, your upload and download times will increase by a factor of 13-14, compared to the 1.5 mbps stream for 576p that you currently use.

Youtube is best placed in terms of expertise and motivation to minimize bandwidth use i.e. to determine the lowest bitrate that yields acceptable quality.