These four grave sins are: Murder (Genesis 4:10), the sin of the Sodomites (Genesis 18:20, 19:13),the oppression of the poor (Exodus 3:7-10, 20:20-22), Injustice to the wage earner (Dt 24:14-15).
And the Lord said, “What have you done? The voice of your brother’s blood is crying to me from the ground.” (Gn 4:10)
Then the Lord said, “Because the outcry against Sodom and Gomorrah is great and their sin is very grave, I will go down to see whether they have done altogether according to the outcry which has come to me.” (Gn 18:20-21)
“You shall not afflict any widow or orphan. If you do afflict them, and they cry out to me, I will surely hear their cry.” (Ex 21-23)
“You shall not oppress a hired servant who is poor and needy, whether he is one of your brethren or one of the sojourners who are in your land within your towns; you shall give him his hire on the day he earns it, before the sun goes down (for he is poor, and sets his heart upon it); lest he cry against you to the Lord, and it be a sin in you.” (Dt 24:14-15)
Some also distinguish the cry of the people oppressed in Egypt, [Ex. 3:7-10]: “The LORD said, "I have surely seen the affliction of My people who are in Egypt, and have given heed to their cry because of their taskmasters, for I am aware of their sufferings.” However, others simply include this as oppression of the poor.