I've been using my Retina iMac (base model 3.5ghz i5 with M290x) with a Dell 27" U2713HM as a secondary display and things were fine. I recently upgraded to a Dell 27" P2715Q 4K display and noticed some graphical quirks on my system. My friend has the same exact setup (base Retina iMac and P2715Q) and hasn't noticed these issues. Unfortunately, he doesn't live nearby, so we can't test all our hardware at once. I've been pulling my hair out trying to troubleshoot, so I thought I'd post here to see if anyone has any suggestions. I'm trying to figure out if there's something wrong with my iMac or the Dell display (or possibly the cables?). For the record, I've already tried resetting the SMC and PRAM, and I also did a clean install of Yosemite, and none of those fixed the issue. I have tried three different Mini DisplayPort to DisplayPort cables. All of them are DisplayPort 1.2: 1) Cable that came with my Dell monitor -- the iMac recognizes that a secondary display is connected in System Prefs -> Displays, but the Dell monitor remains black and no image ever appears. I've tried toggling the "Mirror Displays" box, which I've found to help in the past, but that didn't work either. Surprisingly, I tested this cable and the 4K display with my girlfriend's MacBook Air, and it worked fine. I also tested this cable with my iMac and my old Dell monitor, and that also worked. 2) 3' StarTech cable from Amazon -- the 4K display works fine with this cable, but when I wake my computer/displays from sleep after 5+ minutes (<5 minutes seems fine), the iMac screen will flicker to black twice while the Dell monitor tries to wake. The Dell monitor will then show a light blue color until I login. After I login, the Dell monitor works fine again. Here's a video of what it looks like. (iMac on the left and Dell 4K display on the right. In this video, the iMac background actually switches to a solid gray after the second flicker instead of the frosted desktop background of the login screen, which I've never seen before.) 3) 6' StarTech cable from Amazon -- same results as the 3' cable (scenario 2). Has anyone else seen anything like this? I would really like to figure out if there's a problem with my Dell 4K display or my Retina iMac. I'm willing to try out any other testing scenarios as well. Thanks in advance!