1. Exo 4.22-23 is a nice interplay on ‘the firstborn son.’ YHWH told Moses to tell Pharaoh, “Thus says YHWH, Israel is my firstborn son, and I say to you, ‘Let my son go that he may serve me.’ If you refuse to let him go, behold, I will kill your firstborn son.” A bit tit-for-tat, don’t you think? If you held my firstborn son, I would kill yours. Let go what is mine, and you can still have what is yours.
2. Which is followed by a highly bewildering short story about ‘a bloody husband.’ Moses with his wife and sons were on their way to Egypt, and one night YHWH sought to kill Moses. His wife, Zipporah, then took a knife, circumcise his son, touched his feet (Moses/his son?!) with the foreskin, and YHWH relented. Zipporah exclaimed, “Surely you are a bloody husband to me”, because of the circumcision (Exo 4.24-26).
Of course, the story clearly continued the previous passage about killing the firstborn son, since it involved a murder plan and a son. But still, why on earth YHWH wanted to kill Moses was never clear.
One could perhaps guess that it was related to the circumcision. Moses didn’t circumcise his son, and it was against the earlier command of YHWH to Abraham (Gen 17). And YHWH had something to say (or do) about it: “Any uncircumcised male who is not circumcised in the flesh of his foreskin shall be cut off from his people; he has broken my covenant.” (Gen 17.14, which is another nice interplay; a physically uncut-off male shall be cut off covenantally) So perhaps the passage was inserted by later editor to emphasize the importance of circumcision (seriously, nothing really changed if you skipped 4.24-26 and went straight to 4.27 ff.). Moses forgot to circumcise his son and it almost cost his life. If YHWH didn’t spare Moses’ life, why would he spare yours?
Finally, would Pharaoh ‘circumcise’ his son? Was he willing to cut the Israelites from Egypt?
3. OK, the more I think about it, this is my construal of the two passages. Moses did not let the foreskin of his son go, so YHWH planned to kill him. And this symbolized what would happen to Pharaoh if he did not Israel (YHWH’s firstborn son) go. Somebody would be killed. Not the Pharaoh, though, but all the firstborns. The bloody foreskin is the sign of the covenant, as the blood of the lamb would be in the tenth plague: ‘The blood shall be a sign for you.’ (Exo 12.13)
4. And YHWH ‘passed over’ when he saw the bloody foreskin, as he would ‘pass over’ if he saw the blood of the lamb on the doorpost on the night of the tenth plague, which started the whole tradition of Passover. This cute short story in Exo 4.24-26 then is a sneak preview to the Passover story, so to speak. Moses experienced first what would happen later in the tenth plague.