As the Christian church had more than one gospel available, and particularly when all the gospel writings began to be collected as a set, questions began to arise from comparing them. Some events were recorded in one gospel but not another. Other events were recorded in more than one gospel but not always in the same order. Even in the 100's A.D., careful students of the gospels discussed the differences at some length.
Somewhere around 175 A.D., an early Christian named Tatian put forth the first known harmonization of the gospels. It is known as the Diatessaron, (which roughly means "Fourfold"), a harmonization of the gospels of Matthew, Mark, Luke, and John. Even though Tatian's own later teachings were criticized by the church, still his harmonization in the Diatessaron quickly became a standard work in the early Syrian church, largely replacing the four gospels in common usage there for several centuries.
Tatian's use of Matthew, Mark, Luke, and John as the basis for harmonization is another example showing the unique status of those four gospels in the earliest church, back to the 100's A.D.