You have a good point.
I had a very similar problem to this thread. I replaced my Pi 3 retropie with a Pi 4 and retropie was behaving in a jittery fashion with Emulation Station and the games weren't smooth. I also have a 4K Samsung Smart TV as did the original poster. Although I wasn't using Amiberry, I was using mame, daphne, and jzintv and I figured the same solution that @gordonfreemanjr used would work for me. So I added the five lines to my /boot/config.txt file that he listed in his "IT WORKS" post but lo-and-behold it did not work for me. tvservice -s was still showing a 4K connection when I expected 1080p.
My solution was simple with a twist. Starting from the stock config.txt, I uncommented and modified the following:hdmi_group:0=1 hdmi_mode:0=16
It was necessary for me to add the :0 to get it to work. Now there are no jitters, the motion is smooth, and tvservice -s reports a 1080p connection.