I got a new MG28UQ 28" 4K monitor and I plugged it into my mid 2015 15" retina MBP using a mini display port to HDMI cable. The monitor itself reports it's receiving image at 60Hz, however, Settings > Displays only lists 30Hz under "Refresh Rate". If I lower down the scaled resolution to 1600 x 900, then it starts using 60Hz and the mouse problem goes away.
I'm using an Insignia Mini DisplayPort to HDMI cable that states it can handle 4K.
What am I missing?