I'm also running a 4K TV as a monitor and I had been letting it turn itself off after a certain amount of time not being used. In the morning I was greeted with a black screen that required pressing CTRL + ALT + F1, logging in, and doing various things to get my session back.I did however have to stop using KDE recently as moving from 6x 1080p monitors to 3x 4k tv's presents a situation that seems to aggravate the driver, hardware, as well as the desktop apps down to Qt itself. Being TV's, they're not smart about things like DPMS, and turning them off at night is like unplugging them to the pc. This causes all sorts of weird behavior I'm finding, plasma4.x u14.04+fglrx)couldn't remember which monitor was which to put things back properly when returning, and plasma 5.x (u16.04+ati/mesa) regressed with QT so still cannot deal with displays coming and going well to not simply freak out at times. Again, damn compositors.