I've connected a 4K external display monitor to my 2014 macbook pro via display mini port. Display works fine out of the box.
GPU now has to support 2 displays. That results is increased power consumption (about 20W increase) and excess heat. Just having connected 2 displays is enough to trigger the fans in the computer, which makes it really loud.
What can I do to combat this? I wanted to change the refresh rate of external display to 30Hz, but there is no option in System Preferences to do that. I am running macOS Sierra.