Note that @vince31fr updated the situation here: Fairphone 4 update to iodéOS 7.4
In short, the method @parity and @beetroot followed by unlocking their device (which reformats userdata) and then installing, then relocking basically follows “option 1” that he describes: “clean installing the system”.
Option 2 is to install the update from Recovery, then when still in recovery, do a factory reset.
But the original guidance at the top of this thread hasn’t worked (to install the update from the “Updater” app, then factory reset afterward), so again this means a user needs access to fastboot via a terminal to either unlock the device or just fully reinstall, or to adb sideload the update from Recovery.
Sorry again for the difficulties, this Fairphone firmware bug has been very difficult to solve (effectively this process to reset / upgrade sidesteps the bug to move forward “past” it).
I have heard that Vincent will be elevating the priority to get the FP4 re-integrated in the graphical installer: this would at least allow a user to re-install without a terminal connection to their device (but it still will not preserve user data so if you are able to reinstall or “upgrade + reset” there is no reason waiting for the installer update will help).