2015 Retina iMac Unable to Display 4k@60 Hz on My External Monitor Recently I purchased a 4K monitor for my Mac. I have a MiniDP to HDMI 2.0 converter and a HDMI 2.0 cable. I connected my late 2015 Retina iMac to the external monitor. I was able to get 4k@30hz on my external display. However, I was unable to achieve the 60hz refresh rate. 

After some research I figured that late 2015 iMacs with the thunderbolt 2 ports should support 4k@60hz video output. I can't seem to figure out why I can't get the 60 Hz refresh rate. 

When I open System Preferences, the Refresh Rate drop-down menu is gray. 



Mac Configuration: iMac (Retina 4K, 21.5-inch, Late 2015)
MacOS: Catalina

I connected the same monitor to my 2018 MacBook Pro using a USB-C to HDMI 2.0 cable. My MacBook Pro was able to drive the display at 4k@60hz. 

Thank you for your time and consideration!
The late 2015 21.5-inch Retina 4K iMac is fully able to drive a 4k display at 60 Hz. The monitor you have also lists that it supports 4k at 60 Hz.

The problem here must then either be your HDMI cable or your DisplayPort to HDMI converter.

I would suggest simply trying a different cable/converter.

The easiest to get working is to get a simple mini-DisplayPort to DisplayPort cable, as the monitor has a DisplayPort input. This way you haven't got the converter as an extra point of failure.