pi4:
https://www.raspberrypi.org/forums/viewtopic.php?f=38&t=281606
> You should be able to plug in two 4K displays (or 2x1080p or whatever) and it shoud just work.
> The only difference is that the first port (1) is capable of 4kp60 whereas port 2 can only do 4kp30. If using both you are limited to 30 on both.


https://www.raspberrypi.org/forums/viewtopic.php?p=1728940#p1728940
> HDMI < 2.0 maxes out at 300MHz, and this is described in one part of the EDID.
> HDMI >= 2.0 have to support scrambling as a spread spectrum EMC-reduction step, and that is described in the HF-VSDB (HDMI Forum Vendor Specific Data Block) of the EDID.

4f5f9d02246 firmware: hdmi: HDMI SM clock must not run slower than audio MAI clock See: https://github.com/raspberrypi/firmware/issues/1295