I have two displays running in my current setup: A 27" monitor (it's not 4k, but it's got a somewhat higher resolution than 1920x1080), and a 15" MacBook with the Retina display.
The minor inconvenience I'm running into is that the mouse sometimes gets stuck on the large monitor and won't cross the gap to the laptop display.
As far as I can tell, this is because the edges of the displays are not scaled/mapped to each other's edges. So if the mouse is in the top-right or bottom-right corners of the larger display it's can't cross to the laptop because no shared edge exists there:
I remember running into this issue on a Linux machine and being able to change the mapping/scaling of the display edges so that the mouse can always cross that gap.
So I'm wondering, on El Capitan or Sierra, is there a way to make both displays share a common edge? Either through a setting, config, or even a third-party app?