God's determination to remove Israel from Egyptian slavery will seal the fate of Pharaoh and his people.
"When you go back to Egypt" is a phrase that narratively underscores the overall intentions of God in the actions that will soon commence. Moses will exercise powers given to him by God; some of these will be matched by Egyptian magicians, but the capacity of the latter will soon be exceeded. Pharaoh will be unwilling to let Israel go. Even that unwillingness will be a triumph of God because God will harden Pharaoh's heart. Interpreters have argued that there is a progression in the later narrative from Pharaoh hardening his heart to God hardening Pharaoh's heart. That sequence seeks to soften the affront that the theme of God hardening Pharaoh's heart presents to notions of human autonomy and free will. Some have consequently interpreted Exodus 4:21 as a redactional summation of the endpoint of the narrative. God's endpoint intention to deliver Israel from Egyptian bondage has already been forcefully articulated in the commissioning of Moses. Hardening Pharaoh's heart is a means to that end. In subsequent chapters additional reasons are given for God's hardening the heart of Pharaoh (and the Egyptians), but at the core it serves the objective of Israel's release.
The narrative never permits Pharaoh to argue that God's hardening his heart gives him no choice. He remains the subject of active verbs like "refuse" (4:23), and he is portrayed as admitting that he has sinned (9:27). His agency remains intact.
The narrator starts with two visible givens: (1) Israel has no strength; it is enslaved; (2) Pharaoh is the enslaver; he holds all the power. This is what is seen. But God has also seen the situation. There is a counterforce to Pharaoh's power. The second that Israel gets beyond the Red Sea and is out of Egypt, Israel recognizes how thoroughly God has triumphed on their behalf (15:1). From the point of view of liberated slaves, even Pharaoh's hard and entrenched heart is viewed as the deed of God.
In this section, Israel is termed God's firstborn. When said to Pharaoh, it is to express how precious Israel is in God's sight. One who has read Genesis, however, might be a bit worried. The firstborn in Genesis did not fare well. Here, however, the contrast is between those who are precious to God versus those who are precious to Pharaoh. Everything for Israel depends upon whether God or Pharaoh will triumph.
21 And the Lord said to Moses, When you go back to Egypt, see that you perform before Pharaoh all the wonders that I have put in your power; but I will harden his heart, so that he will not let the people go. 22Then you shall say to Pharaoh, Thus says the Lord: Israel is my firstborn son. 23I said to you, Let my son go that he may worship me. But you refused to let him go; now I will kill your firstborn son.
New Revised Standard Version Bible: Anglicized Edition, copyright © 1989, 1995 National Council of the Churches of Christ in the United States of America. Used by permission. All rights reserved worldwide. http://nrsvbibles.org