First things first - am I supposed to get 4k60p output with the DP port using a DP-HDMI cable? Or can I get it right out of the HDMI port on the 3455?
Please check the motherboard specification what is supported on the HDMI port. The xrandr output let me the assume there is an internal DP-HDMI converter.
Does your external DP-HDMI converter support 4k60p?