Cain’s reaction is not surprising (to us). The man who leads the way and who is the foremost, is shoved to the side. That was not common in his life. He became furious. Understandably so. You have committed yourself, even taken the initiative to sacrifice; you give of something for which you have toiled (Genesis 3:17) and then you have to experience that your little brother (the real number 2) receives God’s attention. That brother also starts to sacrifice, and then gets the priority of God. We can understand how he becomes upset. Especially if you are a man like Cain. Literally it says that something in Cain started to burn and his face fell.
He looked down, to the earth on which he had toiled so hard… Your strong side easily becomes your pitfall. But below you will not find God, for that you must lift your face.
5 but for Cain and his offering he had no regard. So Cain was very angry, and his face fell.