After some time in use the Raspberry Pi 4 showed problems. It seemed like the RPi4 would not boot anymore. The used Operating System placed in three different RPi4’s all gave the same result. No boot.
After considerable technical research, reading the technical specifications, many tests and with the help of descriptions by fellow sufferers on the internet flora it pointed in the direction of failure of the HDMI of the RPi4. From tests and experiences by fellow sufferers it seems that the signal level of the HDMI port on the RPi4 is “tightly” measured.
In connection with monitors/TV screens that require a reasonably high signal level the screen remains “dead”. If you use a “long” HDMI cable, you will lose so much signal that the screen you are using will not activate.

What to do here?
   – Use a shorter HDMI cable, especially when installing the OS for the first time
   – Use a monitor that is satisfied with a lower signal level.
   – Set the HDMI output level of the RPi higher. You can do this by adding the following line to the config.txt file in the /boot/firmware/ folder:
      + config_hdmi_boost=7
By default, this is set to 4 and can be increased to level 9.
   – Let the RPi4 believe that a monitor?TV is connected, i.e. bridge the HDMI-RP4 handshake procedure. For this, include in the config.txt file:
      + hdmi_force_hotplug=1
The handshake procedure is then disabled.
   – The boot file can be set to Computer Monitor or TV. For this, include in the config.txt file:
      + hdmi_group=1 => computer monitor;
      + hdmi_group=2 => “older TV

Don’t set hdmi_safe=1 it will override many previous set options