Issues: I can select resolutions of 4096x2160p @ 30, 29.97, 25, 23.98 and 3840x2160p @ 30, 29.97, 25, 23.98 --- but not at 60fps
Also the same content that used to be 4K HDR under the Rock64 build won't activate HDR on the RPI4.
Add hdmi_enable_4k=1 to config.txt and you should see the 4kp60 modes.
(4kp60 requires higher clock frequencies and scrambling so isn't enabled by default).
HDR isn't currently supported but should come soon.