So that you remember the day of your leaving from the land of Mitzrayim - למען תזכור את יום צאתך מארץ מצרים
Summary:
The connection between יצי"מ and שבת is that פרך becomes וגל through א"ת ב"ש and וגל is בגימטריא 39, referring to the 39 מלאכות, which are forbidden on שבת.
He recites the big Hahlayl on the fourth cup - רביעי אומר עליו הלל הגדול
Summary:
According to ריט"ע nothing is to be drunk after the fourth cup, unless it is necessary, then he should say הלל הגדול on a fifth cup, while according to the רשב"ם, one may drink (even wine) after the fourth cup (and this is the prevailing custom), and according to the רי"ף (and the יש מפרשים), only water may be drunk.