2015 Retina iMac Unable to Display 4k@60 Hz on My External Monitor
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.