[8] In 1278 Jerome was made cardinal priest by Pope Nicholas III, and at some point after 16 May 1279 was assigned the titular church of Santa Pudenziana.

In the event, however, he was unable to attend the chapter for reasons of ill health, as a letter of apology of Pope Nicholas III, written in May 1279, indicates.

As he admitted in his electoral manifesto, Cardinal Masci was extremely reluctant to accept,[12] and indeed he persisted in his refusal for an entire week.

[10] Given the considerable losses to the numbers of the Sacred College in 1286 and 1287, it is not surprising that Nicholas IV quickly proceeded to fill vacancies.

On 16 May 1288, he named six new cardinals: Bernardus Calliensis, Bishop of Osimo (who died in 1291), Hugues Aiscelin (Seguin) de Billon, OP, of the diocese of Clermont in the Auvergne;[14] Matthew of Aquasparta in Tuscany, minister general of the Franciscans since 1287; Pietro Peregrosso of Milan, the vice-chancellor of the Holy Roman Church; Napoleone Orsini; and Pietro Colonna.

In regard to the question of the Sicilian succession, as feudal suzerain of the kingdom, Nicholas annulled the treaty, concluded in 1288 through the mediation of Edward I of England, which confirmed James II of Aragon in the possession of the island of Sicily.

In August 1290 he granted the status of studium generale to the university that King Denis of Portugal has just founded a few months earlier in the city of Lisbon.

[17] His epitaph reads: "Here lies Nicolas IV son of St. Francis" (Hic requiescit / Nicolaus PP Quartus / Filius Beati Francisci).

[18] The 1291–92 Taxatio he initiated, which was a detailed valuation for ecclesiastical taxation of English and Welsh parish churches and prebends, remains an important source document for the medieval period.