The patriarchs of Aquileia proposed themselves as the main, if not the only, local interlocutors of the emperor, as tesserae of a European-sized mosaic, the glue of which were their network of relations and personal ties of loyalty. John appears to be fully included in that picture.
There are not certain news on the family origins and the provenance of this patriarch. It is only sure that John was a ‘fidelis’ of the sovereigns of Saxony, as in that epoch it usually happened for the prelates of Aquileia and, more widely, of the Regnum Italicum, who were often chosen among members of German lineage. His appointment to patriarch of Aquileia most likely dates back to 984. The scarce number of surviving documents of his bishopric evidences firstly the solid relations between the Empire and the position of the Aquileian patriarchy in Italy and Europe between the tenth and eleventh centuries. The advent of the Saxon dynasty had given new impulse to the political and military role of the patriarchs. Whereas Otto I and Otto II favoured Rodoald (963-983), Otto III and Henry II did the same with John. This is documented by the privilege of 18 June 990, which dealt with the supremacy of Aquileia over the bishopric of Concordia and the abbey of S. Mary at Sesto, ad above all the privilege of 28 April 1001 where Otto III granted John and his successors half of the ownership and rights over the castle of Salcano, the village of Gorizia a wide strip of borderland to the Alpine ridges that lies between Friuli and current Slovenia. With a relative frequency John called provincial councils: the ones held at Verona in 995 and Aquileia in 1015 are sure, while the councils in the years 1007 and 1016 are probable. It is reasonable to hypothesize that his long patriarchy was important for the establishment or settlement of some institutions (the canon churches of Cividale and Aquileia, the nun monastery of St. Maria at Aquileia and the monk monastery of St. Martin at Beligna, near Aquileia), the foundation or restructuring of which are most likely due to John. The patriarch John died on 19 June 1019.
For further information see the entry Giovanni IV, patriarca d’Aquileia written by Andrea Tilatti in Nuovo Liruti. Dizionario biografico dei Friulani, 1, Il Medioevo, edited by C. Scalon, Udine, Forum, 2006, 383-388.