In the fourth aliyah, Moshe warns Pharaoh about the plague of the first born, that the first born of the highest Egyptian (Pharaoh) to the lowest, and even the cattle, will die.
After that, G-d gives the first commandment: That of determining the new moon (Rosh Chodesh) and that the months begin with the month of Nissan. (According to Chassidut, everything that is going to happen in a seemingly natural manner, such as birth, sustenance etc., is determined on Rosh Hashanah, in the seventh month, while everything that is going to happen miraculously is determined on Pesach, which is in the month of Nissan, the first month.)
The commandment to take the Pesach lamb is given next, then the eating of Matzo and the prohibition of eating chometz on Pesach.
The Jews were to take the Pesach lamb into their houses four days before they were to slaughter it. The purpose of this was that the Egyptians should see this and ask: "What's going on with these sheep?" The Jews therefore answered without shame that they were going to slaughter them. This was a courageous act because the sheep were gods to the Egyptians!
This strength of character and mesiras nefesh (self-sacrifice) not to be intimidated by the Egyptians brought about the redemption from Egypt.
