Pi HDMI Not Functioning
A common pitfall with the Raspberry Pi board is that the display connected to the HDMI port must be connected and powered before booting the Raspberry Pi. Otherwise, the HDMI output will not be activated. Take special care to sure the HDMI connector is physically inserted all the way into the Pi, especially with the new Micro HDMI connectors on the Raspberry Pi Model 4B.
If this causes you headaches, see this page for code to force the HDMI output to activate regardless of the monitor connection.
Note: Because the Raspberry Pi 4B has two HDMI ports, some HDMI commands can be applied to either port. You can use the syntax <command>:<port>, where the port is either 0 or 1, to specify which port the setting should apply to. If no port is specified, the default is 0. If you specify a port number on a command that doesn't require one, the port is ignored.