These four books provide a quick route to understanding the Gospel according to the four great revivalists of the Great Awakenings: Jonathan Edwards, John Wesley, George Whitefield, and Charles Finney. These are powerful weapons for open air preaching. But I would be careful of Calvinism in Edwards and Whitefield, sinless perfectionism in Wesley and Finney, and Pelagianism in Finney (denial of man’s sinful nature). They were not theologically perfect, but all of them were successful at getting people into the kingdom of God, and preaching the Gospel (justification and sanctification). And getting Christians to live for God.