The other three times the shalsheles appears are all in Sefer Bereishis. All three times are in the middle of a narrative. Here, however, the shalsheles appears seemingly in the middle of nowhere, in the most innocuous and unexpected of places. Moshe has just slaughtered two korbanos as part of the inauguration process of the mishkan, and both times a standard cantillation mark is used for the word "Vayishchat". When he slaughters a third animal, suddenly there is a shalsheles. This begs an explanation.
Let's examine the other three appearances of the shalsheles for a clue. The first time the shalsheles appears is in Parshas Vayeira. The angels have come to rescue Lot and destroy the city of Sedom. They warn him to evacuate the city before sunrise, without regard for his personal possessions. Nevertheless, Lot tarries until the last moment, to the extent that the angels have to forcibly drag him out of his home. The narrative on its own clearly demonstrates that it was difficult for Lot to pick up and flee the city. “Vayismah'mah” indicates that he tarried, and the shalsheles appears on this word, emphasizing that Lot really tarried. This further amplifies what an excruciating struggle it was for him to leave his money behind even to save his life and his family – a powerful comment on the values and priorities of Lot (values which we should eschew).
The second time the shalsheles appears is in Parshas Chayei Sara. Avraham commands his faithful servant, Eliezer, to find a wife for Yitzchak. Eliezer had hoped that his own daughter would merit this match, and was now appointed the agent in fulfilling the dissolution of his own dream. When the time comes for him to pray for the success of his mission, “Vayomar”, this moment is punctuated with a shalsheles, indicating to us the tremendous internal conflict Eliezer experienced. The story would not be truly complete without this subtle remark, indicating the human struggle that Eliezer faced in fulfilling his mission.
The third time the shalsheles appears is in Parshas Vayeishev. The wife of Potifar tries continuously to seduce Yosef, without success. He refuses her advances, “ Vaye'ma'ein”. The shalsheles on this word emphasizes what a herculean feat this was, quite possibly serving as the textual springboard for many of the Midrashic comments regarding this tremendous struggle.
And then we come to the fourth and final time a shalsheles appears in the Torah, during the dedication ceremony of the mishkan. Moshe has just brought two korbanos to inaugurate the mishkan. The word “Vayishchat” appears both times with typical cantillation marks. Now, as Moshe slaughters the third korban, we suddenly have a shalsheles.
Moshe served as the Kohen Gadol during the eight days of inauguration, after which Aharon and his descendants took over the position for all time. The Midrash teaches that Moshe lost this position for resisting too much when Hashem appointed him to be the leader of the Jews. That position was imposed on him, but now this one, which he greatly desired, was taken away. It is this third korban that officially inaugurated Aharon as the permanent Kohen Gadol in place of Moshe. Once again, the shalsheles emphasizes how difficult it was for Moshe to go through with this, what a terrible conflict of emotions he must have experienced as he slaughtered that korban.
As with all the other places it appears, the shalsheles adds a layer of detail and humanity to the narrative, which is fleshed out by Midrashic comments.