In the fourth aliyah, at G-d's direction, Moshe stretches his hand over the sea and the waters return and cover the Egyptians.
The Egyptians wash up dead on the seashore. (Rashi says: to prove that they didn't escape to the other side only to attack again at a later date and be a source of further anxiety).
Then Moshe and the Jews sing the song "Az Yashir" praising G-d (this is also part of our daily Morning Prayer, on page 39 of the Siddur Tehillat Hashem).
Following this, Miriam, Moshe's sister, with the women, sing with their tambourines (which they had brought from Egypt specifically because they knew that G-d would do great miracles for them). (Many women today keep tambourines in their houses in preparation for the final redemption, which we expect momentarily.)
The word used here to mean "sing" -- "Yashir," is in the future tense, indicating that this song will be used again at the time of Techiyas Hameisim - (resurrection of the dead), which will happen along with Moshiach's coming, may it be speedily in our days!
G-d then gives the Jewish Nation some Chukim -- commandments without readily human understandable reasons -- and said that if the Jews keep all of G-d's Chukim, then G-d will not afflict the Jews with any of the diseases of Egypt. G-d then concludes that He is the G-d that heals them.
This alludes to the matter that just as a doctor says "don't eat this thing and it will be well with you," G-d, the one who created us, surely knows what is good for us and tells us how to live, with His Torah.
