i am having trouble finding which HDMI port is rated for what input. The only thing I have found for sure is:
HDMI Port 5 supports a 4k @ 60 Hz input, but only at 4:2:0. It does not support 4:2:2 or 4:4:4 because it does not have the necessary bandwidth. It also doesn’t support HDR.
I don’t know about the other ports. I tried scanninf through the manual but it is short on specifics that i could see.
Additionally I have had issues with my Intel Iris Pro 580 iGPU reliable pushing 4K at all, with random drop outs. I have also seen this happen as a result of the cable used and interference around the cables.
Is the DP to HDMI cable you are using Active or Passive?
Have you tried a shorter cable?