Preconditions:
1) You have to use port hdmi0 which is next to power input, not hdmi1 which is next to 3.5mm jack;
2) Enable "hdmi_enable_4kp60=1" by GUI or CLI
It doesn't work at the beginning:
1) Enable the 4kp60 mode by adding "hdmi_enable_4kp60=1" to /boot/config.txt, or do it under CLI by "sudo raspi-config" -> Advanced Options –> (A9) Pi 4 Video Output.
2) Reboot and completely no video output signal to the monitor, seems like pi4 doesn't even boot to the OS...
Then had a little bit more search, to me the issue was due to "dtoverlay=vc4-fkms-v3d" option inside /boot/config.txt, according to below URL, they won't work together:
https://raspberrypi.stackexchange.com/questions...
So:
1) Comment this line "dtoverlay=vc4-fkms-v3d" inside /boot/config.txt
2) Enable the 4kp60 mode by adding "hdmi_enable_4kp60=1" to /boot/config.txt
3) Reboot and enjoy video output at 4kp60.
