4K video has a resolution of, you better sit down, 4096×2304 pixels. That’s more than four times as much as Full HD (1080p). I don’t think my Mac Mini media center will be able to handle that anytime soon.
If you believe your computer has what it takes, you can try out some of the videos uploaded by YouTube in 4K resolution by selecting “Original” in the same place as you usually select any of the HD resolutions. I tried it out on my brand new MacBook Pro i7 laptop and it was useless – probably also thanks to being run in Flash which doesn’t run that good at all on a Mac.