I will slay thy son, even thy first-born. — The threat was not made until immediately before the tenth plague (Exodus 11:5). It is not recorded in the words which Moses is here directed to use; but the speech of Moses in Exodus 11 is no doubt much abbreviated.
Related Commentaries of Exodus 4:23
Exodus 4:23
23 And I say unto thee, Let my son go, that he may serve me: and if thou refuse to let him go, behold, I will slay thy son, even thy firstborn.