Albert Barnes Commentary Exodus 4:21

"And Jehovah said unto Moses, When thou goest back into Egypt, see that thou do before Pharaoh all the wonders which I have put in thy hand: but I will harden his heart and he will not let the people go." — Exodus 4:21 (ASV)

I will harden - Calamities which do not subdue the heart harden it. In the case of Pharaoh, the hardening was at once a righteous judgment, and a natural result of a long series of oppressions and cruelties.