I run my monitors on the separate mini display ports on my 2015 15 rMBP with dGPU. But mine are not 4K. But, no matter, I prefer to split the bandwidth required between the two display ports.
Since I program I am contemplating a 4K, but not sure I can read the text if it gets any smaller than on a 1920 X 1200 display. I guess the 4K would make the text sharper at 1920 X 1200, like the retina screen. However, am also looking at at the 34 inch curved Samsung.
The text is very readable on a 4k 27 inch monitor that I have. Just need to set the scaling properly. Looks so much crisper than when I was using my old iMac 27" 2009 screen as a second monitor.