How come I can watch 4k on Youtube when my svreen resolution is only 1366x768?

[JoeRaptor]

This makes no sense to me. Is the 4k being scaled down???

 

[Robert Cook]

effectively yes. your GPU is just doing all the work to turn it into 4K for no reason.

 

[unreal9400]

The truth is modern GPU is able to handle 4K content without an issue.
You will notice that 4K youtube content is more sharper compared to 720P even though your display only output 1366*768.

 

[rgd1101]

youtube downscale it to your screen resolution.
you could look at what the real resolution the video playing at the video settings.