The newer, high bandwidth audio signals can not be transmitted over SPDIF connections. TRueHD DTS-HD/MA etc. need the HDMI connection to be transmitted to the receiver. I wonder if some receivers will have issues with an HDMI connection only containing audio signals?
Also, I have a decent Denon AVR that does 4k but I don't use it for video playback except for the cable TV box. I get much better video quality connecting the computer's video card directly to the TV. I use the motherboard's HDMI output to feed audio to the AVR. The two HDMI connections allow the user to do the same even if their AVR is 4k capable.