Q. Who are the saints judging in Rev. 20:4 and concerning what things?
A. There are 2 places where the Lord’s followers are seen in the role of judges. In Matt. 19:28 Jesus said at the time of the 2nd Coming the Disciples would sit on 12 thrones and judge Israel. In 1 Cor. 6:2-3 Paul said saints will judge both the world and the angels. Because of its timing I imagine the basis of the judgment where the world is concerned would be qualification for entry into the Kingdom. We may be assisting the Lord with the 3 judgments described in the Parables of the Servants, the Talents, and the Sheep and Goat Judgment. The judgment of angels would likely concern events of Genesis 6 mentioned in Jude 6.