4:18 Then Moses went [from the region of Sinai] back to [to Midian] Jethro his father-in-law and said [asked (out of respect for Jethro)] to him, “Let me go back to my own people in Egypt to [introduces Moses’ motive for wanting to return to Egypt] see if any of them are still alive.” Jethro said, “Go, and I wish you well.”
4:19 Now the LORD had said to Moses in Midian, “Go back to Egypt, for all the men [presumably Pharaoh’s officials] who wanted to kill you are dead.”
4:20 So Moses took his wife and sons [Gershom (Ex. 2:22) and Eliezer (Ex. 18:4)], put them on a donkey and started back to Egypt. And he took the staff of God in his hand.
4:21 The LORD said to Moses, “When you return to Egypt, see that you perform before Pharaoh all the wonders I have given you the power to do. [note: this is first use of this expression in Exodus…] But I will harden his heart [make him stubborn] so that he will not let the people go.
4:22 Then say [cf. Ex. 4:12] to Pharaoh, ‘This is what the LORD says: Israel [the whole people] is my firstborn [first in rank; Israel has a privileged status among he nations] son [Pharaoh was accustomed to regarding himself and no other as the “son of the gods”],
4:23 and I told you, “Let my son go, so he may worship me.” But you refused to let him go; so [Pharaoh would pay a penalty for refusing to acknowledge Israel as the Lord’s firstborn] I will kill your firstborn son [an explicit allusion to the tenth plague (cf. Ex. 11:1–12:36)].’”
4:24 At a lodging place on the way, the LORD met [in order to prompt Moses to comply with His requirements (cf. Gen. 17:1-27)] Moses and was about to kill him [this statement “represents a common attitude toward severe illness in the ancient world” (BBC, Vol. 1); Moses was likely suffering a severe illness of some kind for his failure to circumcise his son on the eighth day as per Gen. 17:12 (Moses may have failed to circumcise his son out of deference for Zipporah’s wishes); cf. Gen. 17:1-27)].
4:25 But Zipporah [instinctively connected whatever malady Moses was experiencing with their failure to circumcise their son; perhaps she performed the circumcision because Moses was too ill to do so himself] took a flint knife, cut off her son’s [the particular son not specified] foreskin and touched Moses’ feet with it. “Surely you are a bridegroom of blood [refers to circumcision as per v. 26] to me [expression reveals Zipporah’s disgust with the rite of circumcision],” she said.
4:26 So the LORD let him alone [essential that Moses (who was destined to be the great lawgiver to his people) demonstrate complete commitment to God’s law]. (At that time she said “bridegroom of blood,” referring to circumcision.)