Revelation 16: 8 And the fourth angel poured out his vial upon the sun; and power was given unto him to scorch men with fire.
9 And men were scorched with great heat, and blasphemed the name of God, which hath power over these plagues: and they repented not to give him glory.
The fourth angel pours out the next vial on the sun and causes enormous heat to scorch men with fire. But this intense heat not only scorched man but also crops and remember they have been without a lot of water for a while from the last vials. Do you think this changed mans heart, no, they now cursed and blasphemed God. Does this remind you of Pharaoh when his heart was hardened.This plague also caused much pain, like a sunburn but probably about 10 times worse. We need to remember also that God’s people that are still on the earth are protected from all this .