The Fourth Council of Constantinople (Eastern Orthodox) took place in 879-880, while the Fourth Council of Constantinople (Catholic) took place in 869-870.
These two councils represent a break between East and West. The previous seven ecumenical councils are recognized as ecumenical and authoritative by both Greek-literate Eastern Christians and Latin-literate Western Christians. This division led eventually to the East-West Schism of 1054.
In 858, Photius, a noble layman from a local family, was appointed Patriarch of Constantinople, a position of authority rivaling that of Rome. Emperor Michael III had deposed the previous patriarch, Ignatius. Ignatius refused to abdicate, setting up a power struggle between the Emperor and the Pope. In 867, a council in Constantinople pretended to depose the pope, declare him anathema, and excommunicate him.
After the death of Ignatius in 877, Photius mounted the See of Constantinople for a second time. A Council, comprising the representatives of all the five patriarchates, including that of Rome (all in all 383 bishops), was called in 879 and reinstated Photius as Ecumenical Patriarch.
The council also condemned any alteration whatsoever to the original Nicene-Constantinopolitan Creed, thereby condemning the addition of the Filioque clause to the creed as heretical — a view strongly espoused by Photius in his polemics against Rome. Late on, Roman Catholics came to separate the two issues and insist on the theological orthodoxy of the clause. According to Philip Schaff, "To the Greek acts was afterwards added a (pretended) letter of Pope John VIII to Photius, declaring the Filioque to be an addition which is rejected by the church of Rome, and a blasphemy which must be abolished calmly and by, degrees."
Whether and how far the council was confirmed by Pope John VIII is also a matter of dispute: The council was held in the presence of papal legates, who approved of the proceedings, Roman Catholic historian Francis Dvornik argues that Pope accepted the acts of the council and annulled those of the Council of 869-870. Other Catholic historians, such as Warren Carroll, dispute this view, arguing that the pope rejected the council. Philipp Schaff opines that the Pope, deceived by his legates about the actual proceedings, first applauded the Emperor but later denounced the council.
In any case, the Pope de facto accepted the reinstatement of Photius as Patriarch. However later, in the wake of further conflicts between East and West in the 11th century, the council was repudiated.
Though this council has been hailed by some Orthodox Christians as the "Eighth Ecumenical Council" it has not been universally accepted as such, though it is held in high esteem at least as a local council and is referred to as the First-and-Second Council by Byzantine canonists John Zonaras, Theodore Balsamon, Matthew Blastaris and others.
Photius is now considered a saint by the Eastern Orthodox Church; in part for having refused to acquiesce to the decision of the council of 869-870 and to what the Eastern Orthodox consider to have been overweening monarchical aspirations on the part of Pope.