Why are there four Gospels?

Many theologians believe Scripture uses numbers with precise accuracy and real meaning; seven is perfect number and three is the number of the Godhead; forty is the number of testing. A compass has four points: north, south, east and west. We have four seasons to each year: spring, summer, autumn and winter. The Lord divided the field in the Parable of the Sower into four kinds of soil. If four is the earth number, how interesting that the Holy Spirit would give us the four Gospels depicting Christ's earthly ministry.