Disclaimer: None YouTube versions are lower resolution (1280X720) H.265 encoded videos. However, some files are still quite large. At this time, Firefox does not support H.265.
3840X2160 pixels (source)
3840X2160 pixels (source)
3840X2160 pixels (source)
3840X2160 pixels (source)
Stay tuned. There's plenty more where that came from.