Hello. I posted something similar in another thread related to my monitor (p2415q). Let me explain. I have had my mini working good at 4k and 52hz with yosemite and switchresx till now. I moved to El capitan, and it is rejecting the same setup. I disable the new system security to allow switchresx to install the new resolution, I reset, the resolution is enabled and some times it works when I choose it (1080p hidpi, crispy as it used to be in yosemite). The next time I boot the mac, it always goes to black screen, which doesn't make any sense, since it worked the previous session! From that point I have to delete the setup manually from the directory where these setups are stored in order to get the display port working again (in 1440p lowdpi...). I've read a similar post with someone having the same issue with El capitan and another Samgung 4k monitor. Does anyone have a workaround to solve this problem? Thanks in advance. I will stay in yosemite whilst, since I can't bear 30hz and can't see very big improvements in El Capitan, but I would like to solve this though for the future!