Hello! Just bought a 2015 27" iMac and everything about it I love, except for watching 4K videos. Any 4K video I open up and watch in full screen looks pixelated and mushy. I watched other people on YouTube videos testing out 4K YouTube videos in their new 5k iMac and could tell even through the video that their iMac was displaying the content far sharper than mine. Even my 50" 1080p tv shows the same videos sharper through the Chromecast than on my new iMac. Something just doesn't seem right.. 4K videos should look amazing on this thing. Any video I've tried streaming looks bad. There was even a MacWorld article that has a video embedded that says to watch this video of the sun close-up in 4k, you'll be amazed at how it looks on your 5k iMac. It looked like 720p on my iMac.