Why are there four Gospels?

Near the end of the second century C.E., Irenaeus, the bishop of Lyons in Gaul (modern France), had an explanation for why there are (and should be) precisely four Gospels in the New Testament:

Irenaeus says that...heretics had mistakenly assumed that only one or another of the Gospels was to be accepted as scripture: Jewish Christians who held to the ongoing validity of the Law used only Matthew; certain groups who argued that Jesus was not really the Christ accepted only the Gospel of Mark; Marcion and his followers accepted only (a form of) Luke; and a group of Gnostics called the Valentinians accepted only John. All these groups were in error, however, because [argued Irenaeus]

it is not possible that the Gospels can be either more or fewer in number than they are. For, since, there are four zones of the world in which we live, and four principal winds, while the Church is scattered throughout the world, and the pillar and ground of the Church is the Gospel...it is fitting that she should have four pillars...(Against Heresies 3.11.7)

In other words, four corners of the earth, four winds, four pillars—and necessarily, then, four Gospels. [from Bart D. Ehrman's estimable book, Misquoting Jesus: The Story behind Who Changed the Bible and Why, HarperSanFrancisco, 2005, p. 35]

Ah, wondrous the human ways in which the book believed by many to be "the Word of God" took its canonical form....