How can I limit my 4k monitor to 4k resolution and no others, so it doesn't screw up after DPMS?

I’m using drivers 346.47 on my GTX 750 with a 4k dell P2715Q on DisplayPort and two 1080p monitors over HDMI, with the dell as my primary monitor. After the monitors would go to sleep with DPMS (turning them off would produce the same behavior), most of the time when they woke up X would be reconfigured with the DP and one HDMI monitor disabled (like 9 out of 10 times). Setting ConnectedMonitor to “DFP-0,DFP-1,DFP-3” and using CustomEDID (specifying EDID files saved with nvidia-config) in my xorg.conf fixed that, but now, despite having only a single metamode:

Option “metamodes” “HDMI-0: 1920x1080 +5760+0, DVI-I-1: 1920x1080 +0+0, DP-1: 3840x2160 +1920+0”

Option “IncludeImplicitMetaModes” “off”

After restarting X, or after DPMS puts the monitors to sleep, the 4k monitor still comes up scaling to 2560x1440, rather than at its full resolution. (The 1080p monitors come up at 1080p.)

Is there any way to really and truly limit X to just 3840x2160 on my 4k monitor, and no other resolutions, so I can finally put my monitors to sleep without having to reconfigure the 4k monitor every time it wakes up (nvidia-settings can restore it to native resolution on the fly, i.e. by choosing that, pressing “Apply”, and then saying yes to the dialog which asked if I want to keep it)?

I have a similar issue. I have two Samsung U32D970 4k monitors plugged into a GTX970 via DisplayPort cables. They work fine until they go to sleep via DPMS. When they wake, one of the monitors will usually come up in 2560x1440 resolution. If I unplug the monitor from the card and plug it back in, then it’s back to the native resolution. I did notice via nvidia-settings, that when a monitor is stuck in 2560x1440 resolution, nvidia-settings reports the DisplayPort connection link as 2 lanes rather than the normal 4 lanes. This behavior is consistent across computers (using the same model card) and operating systems (Windows 8.1 and Ubuntu 15.04) with the latest nvidia drivers available on both (346.47 on Ubuntu, 347.88 on Windows 8.1).