@Moem Thanks. I downgraded to Android 11 before running the UBports installer, and that was successful. It wasn't all first-time-right (see above, had to run fastboot as root), but it was easy enough to find and overcome the obstacles using this forum.

@kevin-lhotellier I'm in the same situation (or similar).

With respect to the command "fastboot flashing unlock" and its response "waiting for any device", and then waiting indefinitely...: what worked for me is to run this command as root in Ubuntu. I tried this after reading a suggestion to run the command 'as Administrator' in Windows, on the Fairphone forum.

With repect to the instruction in UT for FP4: "Your device must be running the Fairphone OS version of Android 11 before installing another operating system. With a previously installed custom ROM, it won't work!", I wonder if a downgrade from Android 13 to 11 is at all necessary. Android 13 is hardly a custom ROM, isn't it?

Any help or experiences regarding this last question?

Thanks!