A:Answer Maybe check your source, if you are looking at a streaming device that offers 4K and you are using 4K HDMI cables with 18gbps then you will see that the TV does full 4k/HDR regardless of what HDMI port that you use. Ports 2 and 3 are the only ports that are full 4k / 60fps. The rest are at 4k/ 30fps which will work perfectly for say 4k Blu-ray players that only does 4k/24fps for playing back disks. For 4k gaming systems such as the Xbox One X, or some media streaming at 4k such as Netflix 4k , Amazon 4k, you would want to use HDMI ports 2 and 3. If you have for example a Roku Ultra 4k and no Netflix/Amazon 4k service, then your regular Netflix/Amazon HD will be upscaled to 4k at 2160p.
Note: PS3 and PS4 do not stream or play anything at 4k and will not be upscaled to 4k which is probably why you don't see the 2160p (Only PS4 Pro). To check this try using your tv's independent apps and the set will upscale your internal streaming media say from Netflix, Amazon, Crackle, etc to 4k.