Tatian was a Pythagorean Therefore the δια τεσσάρων συμφωνία Necessarily Involved the Harmony of Two Gospels Not Four

One should not suppose, as some have expounded the text (1 Cor 7.39), that when Paul says the wife is bound to her husband he means that flesh is involved in corruption. He is attacking the notion of the godless men who attribute the invention of marriage directly to the devil, a notion which dangerously blasphemes the lawgiver. I believe Tatian the Syrian made bold to teach these doctrines. At any rate he writes these words in his book On Perfection According to the Saviour (Περὶ τοῦ κατὰ τὸν σωτῆρα καταρτισμοῦ):

While agreement (συμφωνία) to be continent makes prayer possible, intercourse of corruption destroys it. By the very disparaging way in which he allows it, he forbids it. For although he allowed them (Adam and Eve) to come together again because of Satan and the temptation to incontinence, he indicated that the man who takes advantage of this permission will be serving two masters, God if there is 'agreement,' (συμφωνίας) but, if there is no such agreement (συμφωνίας), incontinence, fornication, and the devil.

This he says in expounding the apostle. But he falsifies the truth in that by means of what is true he tries to prove what is untrue. We too confess that incontinence and fornication are diabolical passions, but the agreement (συμφωνία) of a controlled marriage occupies a middle position. [Clement Strom. 3.13]