From his Explanatory Notes Upon the New Testament (1754)
St. Mark in his Gospel presupposes that of St. Matthew, and supplies what is omitted therein. St. Luke supplies what is omitted by both the former: St. John what is omitted by all three.
St. Matthew particularly points out the fulfilling of the prophecies, for the conviction of the Jews. St. Mark wrote a short compendium, and yet added many remarkable circumstances omitted by St. Matthew, particularly with regard to the Apostles, immediately after they were called. St. Luke treated principally of the office of Christ, and mostly in an historical manner. St. John refuted those who denied his Godhead: each choosing to treat more largely on those things which most suited the time when, and the persons to whom, he wrote.
Funny there is no mention of Q, but I guess we can’t all be post-liberal re/de-constructionists.