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.