How can I use a 4K display as my primary monitor?

Here's a small update to the above use report; after continued use and additional restarts, the resolution scaling choices have returned to the Systems Preferences Display panel but still top out at 1080p. When connected to the Mac, I have also checked the status of the Acer monitor using its external controls (front panel buttons) and the read-out they give is that the monitor's native resolution is 1920 x 1080. Resetting the monitor's settings from the control panel does not reset it to its true native resolution of 3840 x 2160. Acer first line support says it's a plug & play monitor whose resolution can't be reset -- but it appears that has indeed happened as a result of the interaction with Mac OS X El Capitan. Acer has escalated the issue to "level 2" and I'll be pursuing that line of inquiry in the coming week.

This indeed expands the list from 4 or 5 to 13 choices; unfortunately all the added choices are lower resolution than the current ceiling of 1080p (1920 x 1080). Thank you for making me a little smarter about my problem. I still believe that somehow the monitor has been physically reset to 1080p but I have no idea how it happened -- if it is a plug & play as Acer and their documentation state, it should not have gotten reset.

Since my last post, I've discussed the matter with Acer's top level (3rd level) support and we can find no issues with the Acer hardware. So the mystery remains: why would El Capitan support 3840x2160 and then later no longer support it? When I chose to change the resolution from 4K to 2K, I had a number of choices, e.g., 2560x1440, besides coming down to 1080p but now 1080p is the highest resolution available. If I had never been at 4K, I'd suspect the issue was my Mac mini which is several years old. Personally, I think it's a driver issue but have no means to look into that. Absent a miracle, I'll move to Windows 10 for support of this monitor.

Thank you for providing further insight on this problem.

Well, I'm 99% sure that the display first came up in 4K mode -- I very much recall that the icons were quite small (smaller than they were on the HDMI-connected 1080p TV that I used previously) and that led me to go to Systems Preferences - Display, and see what other resolution choices there were -- and I was surprised to see that there were intermediate choices between 3820 x 2160 and 1920 x 1080. So, I chose 1080p for reasons making the display more readable in the short term.

The connection I used for this initial use was Display Port using a full port at the display end and the Mini Display Port on the Mac side.

I looked up the history of my Mac Mini -- I purchased it in June 2010.

Return to 4K for the subject Acer monitor: I brought in another 4K display (HP) and hooked it up to my Mac Mini via HDMI, and connected the 4K Acer as a 2nd display. The HP that was connected via HDMI and came up in 1080p resolution, while the Acer (connected via Display Port) and came up in 4K resolution.

And I could see display preference choices that I was wanting to get back to...

The bad news is that my 2010 vintage Mac Mini can only drive the display at 30 Hz (vis-a-vis 60 Hz which is optimal for the display). The resulting performance of the 4K display is quite slow, e.g., dragging a window's corner to fill the screen. The real solution to the problem that started this thread is to bring a current Mac Mini model (or equivalent).