Yeah... 4K videos are great for testing out pretty much any screen -- especially televisions, even if they aren't 4K -- a good 'best-case-scenario' of screen quality.
Bookmarked. I'll keep this link handy for future gawkers.
On those same lines, this game does a great job of showing off the screen as well. Lots of cool particle effects and it's fast. (It has a free demo) Riptide GP2 app for Windows in the Windows Store