I am not sure I understand why you would need the serial console…
You can access the same info via SSH:
I was able to boot the RPi4B without the Dummy Monitor plug. I noticed the config file at /boot/config.txt include a few lines containing HDMI. After digging through the RPi config documents I found a setting that seems to work.
So I added hdmi_safe=1 and it seems to boot OK with and without a HDMI monitor.
Once booted, I can access the console via SSH.