Hello piero,
I ended up trying to install from Windows 11.
Thanks for your answer, but in fact the web-installer didn’t work. Reason why it didn’t work was that Windows 11 didn’t recognise the fairphone 4’s adb-driver and unlocking of the bootloader didn’t work.
The fairphone told me that the bootloader was still locked. I had to follow the step-by-step-guide using the driver-guide from GitHub - fawazahmed0/Latest-adb-fastboot-installer-for-windows: A Simple Android Driver installer tool for windows (Always installs the latest version) (Thanks to him!). Using this tool I had the proper adb-driver to use the mentioned commands to unlock the bootloader, following the upload of /e/OS (official version 14) and locking the bootloader finally.
After all that the fairphone 4 (with /e/OS) was not known by Windows 11 and I had to remove the former installed driver (not the device only) by uninstalling, and searching for the correct device-driver.
After some extra hours of investigations I have what I wanted to have: my /e/OS-fairphone.
best regards and thanks for your help 
beta-L-user