By default, integrated graphics is the go to output for non GPU intensive operations. in essence the 4K is another screen.. so is not seen as a GPU hungry application so will try to run using Integrated graphics. Problem here is the 4K will not run using integrated graphics so you have a catch 22.
the way you get it to play ball, is to disable the IGPU in the BIOS, then disable it in your device manager. That way your dedicated GPU - in your case the 1080ti will be the only GPU it has on offer and will push the signal that way. This does mean that your device will not be using your IGPU on low GPU applications and will use more power and not be running in power saving mode, but it will get your 4K working