Sissinios 4 was bishop of Herakleia in Thrace (Europa); he attended the Third Council of Constantinople (the Sixth Ecumenical Council) in 680 and 681 and was present at all eighteen sessions: Riedinger, pp. 16-823 (= Mansi XI 209-669). In the subscriptions to the statement of the faith, which condemned monotheletism, and to the Council, he is styled Σισίννιος ἐλέει Θεοῦ ἐπίσκοπος τῆς Ἡρακλεωτῶν μητροπόλεως τῆς Εὐρωπέων ἐπαρχίας: Riedinger, p. 780, lines 11-12, p. 823, lines 7-8, cf. p. 891, lines 30-31 (= Mansi XI 641, 669, cf. 689). In the lists of those attending each session of the Council he is styled Σισινίου ἐπισκόπου Ἡρακλείας Θρᾴκης: Riedinger, p. 16, line 22, p. 28, line 26, etc. (= Mansi XI 209, 217, etc.). At the eighth session (7 March 681) he declared his acceptance of the doctrine of Two Wills and Two Energies as set out in documents from pope Agathon (Agatho 1) and the Roman Council: Riedinger, p. 198, lines 1-5 (= Mansi XI 336-337). He was one of those who signed the copy\n of the definition of the faith sent to the bishop of Rome, Agatho 1, on behalf of the bishops under him as well as himself: Riedinger, p. 891, lines 30-31 (= Mansi XI 689) (ὑπὲρ ἐμαυτοῦ καὶ τῆς ὑπ' ἐμὲ συνόδου).

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