Stephen IV was elected on June 22, 816, after the death of Pope Leo III. His brief pontificate (just over seven months) was nevertheless institutionally foundational: he made the unprecedented journey to Reims in October 816 to personally crown and anoint Emperor Louis the Pious — Charlemagne's son — establishing the foundational ritual that papal coronation alone constituted valid imperial succession in the Carolingian succession.
The Reims coronation of October 816 was the first occasion on which a Carolingian emperor received the imperial crown from the pope's hands outside Rome — Charlemagne having been crowned at Rome in 800. The precedent that the pope personally bestowed the imperial diadem (rather than confirming a Frankish royal coronation) became the foundational legal-theological basis of medieval imperial-papal relations.
Stephen also obtained from Louis the Pius the foundational 'Pactum Hludovicianum' confirming and expanding the territorial sovereignty of the Papal States — a Carolingian renewal and amplification of Pippin's foundational Donation of 756. He died on January 24, 817, having completed in just seven months the foundational institutional confirmation of the Franco-papal alliance for the next two centuries.