Why is it wrong to preach only from the four Gospels and ignore Paul?
Preaching only from the four Gospels ignores that Christ's earthly ministry was still under the Mosaic system, and His death, burial, and resurrection had not yet happened. The gospel of grace could not be preached until God revealed it after that finished work, so ignoring Paul omits the completed gospel God revealed.
Les teaches that everything Christ said and instructed during His earthly ministry was in compliance with the Mosaic system, not the gospel of grace. That gospel could not yet be preached because Christ had not died; the death, burial, and resurrection had not taken place. Les points to Luke 18:31, near Caesarea Philippi, where Jesus told the Twelve that all things written by the prophets concerning the Son of Man would be accomplished: He would be delivered to the Gentiles, mocked, spitefully entreated, and spitted on. Since that was still future, the work was not completed. You cannot preach something until God reveals it, and He had not yet revealed it. So preaching only the four Gospels while totally ignoring the Apostle Paul wrongly skips the completed work and the revelation of the gospel of grace.