Quattuor abhinc annos (Latin for "four years ago") is the incipit of a letter that the Congregation for Divine Worship and the Discipline of the Sacraments sent on 3 October 1984 to presidents of episcopal conferences. concerning celebration of Mass in the Tridentine form.
The Congregation thereby granted diocesan bishops an "indult" to authorize celebration of the Tridentine Mass according to the 1962 Roman Missal by specified priests and groups of the faithful who requested it.
The permitted Tridentine Masses were to be in full accord with the 1962 Missal and in Latin.
An important condition for granting the requests was "that it be made publicly clear beyond all ambiguity that such priests and their respective faithful in no way share the positions of those who call in question the legitimacy and doctrinal exactitude of the Roman Missal promulgated by Pope Paul VI in 1970."
Diocesan bishops refused many requests by people whom they considered not to have met this condition. But authorization was in fact granted either by diocesan bishops or directly by the Holy See to many priests, parishes and priestly societies, who could then use the older forms of the Roman Rite either exclusively or only on occasion. The priestly societies included the Priestly Fraternity of St. Peter, the Institute of Christ the King Sovereign Priest, and the Personal Apostolic Administration of Saint John Mary Vianney. These used the "Tridentine" liturgical books exclusively, not only for celebrating Mass but also for the other sacraments and rituals and for the Divine Office. Individual priests and communities belonging to religious institutes also received the same authorization. There were such cases among the Fraternity of Saint Vincent Ferrer, the Institute of Saint Philip Neri, the Canons Regular of the New Jerusalem, the Canons Regular of Saint John Cantius, the monasteries of Sainte Madeleine du Barroux and Sainte Marie de la Garde. Various diocesan clergy also availed of the document's provision. See Communities using the Tridentine Mass for a list of priestly societies and religious institutes which celebrate the Tridentine Mass.
Groups such as the Society of St. Pius X, who maintained that they needed no permission to celebrate the Tridentine Mass decried the document and referred rather derisively to Masses celebrated with the Quattuor abhinc annos authorization as "Indult Masses". Several of these groups, such as the Society of St. Pius V, preferred to celebrate Mass according to pre-1962 editions of the Roman Missal.
The view that use of the earlier form of the Roman liturgy had never been formally abrogated  was authoritatively confirmed by Pope Benedict XVI, who declared that permission to use it (which can be granted by the priest in charge of the church) is required only for public celebration.
In his motu proprio Ecclesia Dei of 2 July 1988, deploring the action of Archbishop Marcel Lefebvre in consecrating, two days before, four bishops against the express prohibition of the Holy See (see Ecône consecrations), Pope John Paul II called for "wide and generous application" of the Quattuor abhinc annos directives.