Just to give some perspective on what exactly the size of 4K is, the ideal screen size for a 4K video is 25 feet and IMAX movies are projected through two 2k resolution projectors. So what this means is it will look *really* crisp on your 15″ laptop screen.
In order to try this feature out, on any video that supports it pick “Original” from the format selection popup on the youtube player. Since there are so few videos currently in this format a playlist has been created to feature them.
It looks like YouTube updated their specs on what they support again changing it from 4096×3072 to 4096×2304 or 2304p. Still impressive, but it’s starting to lose the coolness a little. Thanks to Quboid for the tip.