The Arizal (Rav Yishak Luria of Safed, 1534-1572), in Sha’ar Ha’kavanot, writes that the Sofer (scribe) who is writing the text of the Tefillin must ensure to write all four Parashiyot (sections) at one time, without any interruptions. This means that the Sofer should not write two Parashiyot in one sitting and the leave the other two for the next day or even for later that same day. Rather, he should write all four Parashiyot in a single sitting, without making any interruption. The Arizal emphasizes that one should not make even verbal interruptions by speaking in between Parashiyot, and certainly one should not make an interruption in the middle of a Parasha.

If the Sofer is elderly and does not have the strength to write all four Parashiyot in a single sitting, then, the Arizal says, he should at least write the first three Parashiyot in a single sitting, and write the fourth one later. If even this is too difficult, then he should write the first two Parashiyot in one sitting and the last two Parashiyot later.

(The Arizal added that if one did make an interruption, he can correct the mistake by rewriting the first Pesukim of the Parashiyot he had already written, and then writing the final Parashiyot, as he may then be considered as having written all four Parashiyot in one sitting. Some later Aharonim, however, question the validity of this practice.)

It must be noted, however, that this requirement applies only on the level of "Hasidut" – a special level of piety, as opposed to strict Halachic obligation. And therefore if a Sofer suspects that if he tries writing all the Parashiyot in one sitting he will grow tired and be unable to properly concentrate on his work, it is far preferable to do the work in installments. In such a case, one should sacrifice the Misva Min Ha’mubhar (special standard of Misva performance) in order not to compromise the quality of his work.

When a person goes to purchase Tefillin from a Sofer, it is important to ask pointed questions in order to ensure that he receives the highest-quality Tefillin. And one question that should be asked is whether the four Parashiyot were written all together in a single setting. There are, Baruch Hashem, many young Soferim who are capable of writing all four Parashiyot in a single setting, and given the Arizal’s insistence on this method of writing, one should therefore endeavor to purchase Tefillin that were written in this fashion. However, as mentioned, if the Parashiyot were not written all at once, they are nevertheless valid for use.

Summary: One should try to purchase Tefillin whose four Parashiyot were all written in one sitting, without interruption, though if they were written in installments the Tefillin are nevertheless valid for use.