Youtube blows past 1080p HD with 4K videos...
If you have a fast Internet connection, or a lot of patience:
Take a look at this.
I'll have to try this at home, here at work I'm not seeing much difference between 1080 and 4k
Edited to add: Doh! Never mind. Full resolution is 4096 x 3072. That's just a tad above what my monitor/graphics card can pump out.