In the introduction to his collection of canon law documents (canonical collection), Dionysius Exiguus (c. 525) discusses how the Greek canon is the same as the Latin regula, then moves on to a discussion of the (then) four main councils:
Now, the canons of the general councils begin from the times of Constantine since in the preceding years when persecution was raging, opportunity for teaching the people was least given. Then Christianity was divided into various heresies because there was no freedom for the bishops to come together into one except in the time of the abovementioned emperor. For he gave the opportunity to Christians to gather freely. Under this also the holy fathers, coming from all the globe of the lands in the Nicene Council handed down the second symbolum (henceforth ‘creed’) after the Apostles joined to the evangelical and apostolic faith. Amongst the rest of the councils, there are four venerable synods which principally embrace the whole faith just like the four Gospels and as many rivers of Paradise.
Of these, the first was the Nicene Synod of the three hundred and eighteen bishops, carried out with Constantine Augustus ruling. In this one, the blasphemy of the Arian falsehood was condemned, which concerning the inequality of the Holy Trinity that Arius indeed asserted, the holy synod defined that God the Son is consubstantial with God the Father through a creed.
The second synod of 400 fathers under Theodosius the Elder was gathered at Constantinople which, condemning Macedonius who denied that the Holy Spirit was God, demonstrated that the Holy Spirit is consubstantial with the Father and the Son, giving the shape to the creed which the entire confession of the Greeks and Latins proclaim in the churches.
The third synod is the first of Ephesus, of 200 bishops, produced under Theodosius the Younger, which justly condemned in an anathema Nestorius who was asserting that there were two persons in Christ, showing that there is one person of the Lord Jesus Christ in two natures.
The fourth synod, that of Chalcedon of 630 priests (sacerdotes), was held under the princeps Marcian. In it, one judgement of the fathers condemned Eutyches, an abbot of Constantinople who was a defender of the one nature of God the Word and the flesh, and a certain Bishop of Alexandria and Nestorius himself again with the rest of the heretics. The same synod proclaimed that Christ God was so born from the virgin that we confess in him the substance of the divine and human nature.
These are the four principal synods that proclaim most fully the teaching of the faith but also, if there are councils which the holy fathers, filled with the Spirit of God, sanctify after the authority of these four, they would remain with a sturdy strength, whose established deeds are contained in this work. (My trans. from Vat. lat. 1337, fol. 1r-v)
Dionysius’ main concern is not, of course, their doctrinal rulings, although he does include the symbolum of Nicaea amongst his texts. His concern is their canons, their regulations for church life, all of which he freshly translated out of Greek into Latin alongside the regulations from various local church councils and the document called the ‘Canons of the Apostles’. Later on he compiles a collection of papal letters (thus my interest in him). This letter collection is appended to that of the canons.
One quick reflection upon what is an intrinsically interesting document. All four councils were called by emperors, all of whom Dionysius names — Constantine, Theodosius I, Theodosius II, Marcian. Indeed, he states that such general councils would not have been possible before Constantine because of boiling persecution (persecutione feruente). This point is made elsewhere. It is worth thinking on. Today, we have no emperor to make safe the roads and compel bishops to come by edict. But back then, that was the only way to get as many together to qualify as worldwide (ecumenical). Note also that Dionysius leaves room for more councils!
It is also worth remembering that local synods are attested frequently before Constantine, such as the deposition of Paul of Samosata at an Antiochene synod in the 200s, or various local synods in Rome. Or others that escape my mind but likely happened. From the Council of Jerusalem in Acts, church leaders have always sought to gather together to discuss, pray, and debate thereby discovering what ‘seemed good to the Holy Spirit and to us’ (Acts 15:28).