Must one repeat the Friday Prayer as four rak‘ahs or two, if one remembers after performing it that one did not pray Fajr?

You are only required to make up the Fajr prayer because the order between the two prayers is desirable (mustahabb) and not obligatory (wajib) according to the Shafi'i school, which is the preferred opinion. And according to the opinion that considers it obligatory, it is waived by forgetfulness.