People are brilliant at claiming they see a difference when they know which TV is 4K and which is 1080P. I haven't found anyone yet who actually could when they didn't know.
Try looking at uncompressed animated features. An uncompressed feature at natively animated at 4K and displayed at 4K is notably different from a 4k or 1080p one on a 1080p display. A big leveller is the compression used in broadcasts.