Let’s say someone has 3 parshiyos that are kosher but not mehudar and one parsha that is considered to be mehudar. Is there any milah in this? Can the Tefillin be called Mehudar at all? What if someone has an option to change one parsha (let’s say the last of tefillin shel rosh) to a “mehudar” parsha is there any ma’alah in this even though the others are still not considered “mehudar”.
With regard to the parshios of tefillin, there is certainly only one mitzvah, and if one of the pashios is not “mehudar” the entire tefillin is not mehudar.
This is more certain than the question of the four minim, where the Torah instructs the taking of each min individually, and the Rambam rules (7:6) that taking each one individually fulfills the mitzvah.
In the case of tefillin, there is room to doubt whether there is any advantage of having one of the pashios with a hiddur, where the others are without it.