The poster in your other thread is not going to be proven wrong if you have more than one HDMI 2.0+ input on your TV. He used the word "may," to indicate it's a possibility that you have only a single HDMI 2.0+ input. It's not uncommon to only have a single input that supports the full feature set of the TV. Manufacturers save cost this way.
His point seemed to be that if you're just going to plug into inputs without first reading your manual or verifying which port you are actually supposed to be connecting your 4k device to, you need to try every port until you find one that works correctly. If none of your inputs end up working without issue, it's possible that one or both of your devices are too close to either end of compliance in the HDMI spec and may not happily work together.
Are you using a very cheap, flimsy HDMI cable, or one that is overly long, such as 25 feet or longer? Any adapters or couplers that could be causing signal loss or corruption, or running your HDMI cable past any sources of strong interference such as AC wall adapters?