I recently bought a 4K monitor Dell P2415Q of revision A04. It is connected to a Mac mini 2018 by HDMI cable. At first, every operation displayed on this monitor seems a bit sluggish. From “System Information” -> “Graphics/Displays”, I found the refresh rate of this monitor is only 30Hz (instead of 60Hz).
The solution is from this Dell doc: unplug all video cables -> power on this monitor -> select “HDMI(MHL)” as input -> press the third button (indicated by a green-color arrow) for 8 seconds -> in the pop-up window, disable “HDMI 1.4”, which will enable HDMI 2.0.