HDMI and Displayport (DP), typically you see 1.2 version, have different rates of data transfer. So, for instance, on my 1440p 144hz monitor, I need a DP1.2 in order to get the max speed. With HDMI, I can only do 60hz and even then, most games have some wavy artifacts. There is a huge difference between the two (I just tested this with The Witcher 3).
greens is definitely accurate: you do not want to use a monitor at a lower resolution. It will only look okay if you use a portion of the screen (so that the unused pixels are left black, unused). I've run 1080p on my 1440p and it looks real bad, blurry.
I'm using MSI's 1070, OC to 2088mhz, and I get between 60 and 90 fps on The Witcher 3 with settings at Ultra (grass density at high), AA off, Hairworks off. I think a smoother framerate (60+) at 1440p is to be desired over 4k. Of course, this is all a matter of preference. My dream is not even to have 4k, but to do ultrawide (3440x1440p). Though not all games support the 21:9 aspect ratio.