On the Development of the Fourfold Gospels

Yes, there is now enough time for the development of the N.T. The following is just provisional. There is no evidence whatever of the set of four Gospels before Irenaeus. There were several Gospels, but no person or group used a set of four with equal authority. Geographical distribution is often mentioned in the textbooks, but several scholars have pointed out that there is no evidence for this. At most, one book was better known in a place where one party or sect was dominant. Any or all of the four could have been revised before they were promulgated as a set. The evidence confirms your picture of a set of four shortened Gospels made by abridging the books distinctive of each sect (which means careful selection of what to leave out). The ending of John was adjusted so as to make it read like the conclusion to the set of four. The set of four was probably published in Rome in about 170 A.D. It was in Greek and only in Greek. Translations into other languages were delayed for reasons uncertain. The older long single Latin Gospel, in the edition of the Diatessaron and probably also in an edition like the one used by Justin, was used for all preaching and teaching in Latin and for all liturgical purposes in Latin. Serious books were written in Greek in Latin-speaking areas and these quoted the Fourfold Gospel. No-one writing in Latin used the set of four before Tertullian, and he read them in Greek and did his own translating as needed. There was no use at all for any purpose of the set of four in areas where the language was Aramaic or Armenian or Georgian for centuries afterwards. (I don’t know yet about areas where Ethiopic or Coptic or Arabic or Gothic were used. However, the indications are that the pre-Islamic Arabic Gospel was a single book).