But look at plagues 2 and 4. After those ones, Exodus tells us that Pharaoh hardened his own heart. As I recall, Exodus only says that the Lord hardened Pharaoh’s heart after 4 plagues. After the other 4, it was simply “Pharaoh’s heart was hardened.”
Also note that Pharaoh’s heart started out hard and was only softened when the plagues struck. The mechanism by which God hardened his heart was through removing the plagues.
Exodus 4:21-23
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. But** I will harden his heart so that he will not let the people go. **22 Then say to Pharaoh, ‘This is what the LORD says: Israel is my firstborn son, 23 and I told you, “Let my son go, so he may worship me.” But you refused to let him go; so I will kill your firstborn son.’ "
Exodus 7: 1-5
1 Then the LORD said to Moses, “See, I have made you like God to Pharaoh, and your brother Aaron will be your prophet. 2 You are to say everything I command you, and **your brother Aaron is to tell Pharaoh to let the Israelites go out of his country. 3 But I will harden Pharaoh’s heart, and though I multiply my miraculous signs and wonders in Egypt, 4 he will not listen to you. **Then I will lay my hand on Egypt and with mighty acts of judgment I will bring out my divisions, my people the Israelites. 5 And the Egyptians will know that I am the LORD when I stretch out my hand against Egypt and bring the Israelites out of it.”
Plague 1: Blood
Exodus 7: 22 But the Egyptian magicians did the same things by their secret arts, and Pharaoh’s heart became hard; he would not listen to Moses and Aaron,
just as the LORD had said
Plague 2: Frogs
Exodus 8:15 15 But when Pharaoh saw that there was relief, he hardened his heart and would not listen to Moses and Aaron,
just as the LORD had said.
Plague 3: Gnats
Exodus 8:19 The magicians said to Pharaoh, “This is the finger of God.” But Pharaoh’s heart was hard and he would not listen,
just as the LORD had said.
Plague 4: Flies
Exodus 8:32 But this time also Pharaoh hardened his heart and would not let the people go.
Plague 5: Livestock
Exodus 9:7 Pharaoh sent men to investigate and found that not even one of the animals of the Israelites had died. Yet his heart was unyielding and he would not let the people go.
Plague 6: Boils
Exodus 9:12 **But the LORD hardened Pharaoh’s heart **and he would not listen to Moses and Aaron, just as the LORD had said to Moses.
Plague 7: Hail
Exodus 9: 35 So Pharaoh’s heart was hard and he would not let the Israelites go,
just as the LORD had said through Moses.
Plague 8: Locusts
Exodus 10:1-2, 20
1 Then the LORD said to Moses, “**Go to Pharaoh, for I have hardened his heart and the hearts of his officials so that I may perform these miraculous signs of mine among them 2 that you may tell your children and grandchildren how I dealt harshly with the Egyptians and how I performed my signs among them, and that you may know that I am the LORD.” **20 But
the LORD hardened Pharaoh’s heart, and he would not let the Israelites go.
Plague 9: Darkness
Exodus 10: 27 But
the LORD hardened Pharaoh’s heart, and he was not willing to let them go.
Plague 10: Firstborn
Exodus 11:1, 9-10
1 Now the LORD had said to Moses, "I will bring one more plague on Pharaoh and on Egypt. After that, he will let you go from here, and when he does, he will drive you out completely.
9
The LORD had said to Moses, “Pharaoh will refuse to listen to you—so that my wonders may be multiplied in Egypt.” 10 Moses and Aaron performed all these wonders before Pharaoh, but the LORD hardened Pharaoh’s heart, and he would not let the Israelites go out of his country.