John IV of Gothenland (The Kalmar Union)

John IV was king of Gothenland. His reign would see a marked increase in the land nominally held by Gothenland in Pomerelia.

Only six years when his father Eric III died the young king was placed under a regency of his mother, Matilda of Holland and Benedict Porse, the elderly and genial Duke of Eyland (Öland).

In November 1370 the long-reigning Casimir III of Poland died, his only heir was his nephew John I of Hungary leading to both kingdoms being ruled in union. In an extraordinary conference in Visby in early 1371 envoys from Gothenland, the Knights and the Holy Roman Empire agreed a league to oppose John of Hungary's ambitions in Poland. Within Poland itself several nobles, especially those from the Piast dynasty took up arms. Gothenland's armies raided western Poland liberally, seizing Slupsk, Betowo and Bydgoszcza.

At Torun in August 1377 the League was crushed. Whilst both the Knights and Gothenland were allowed to hold onto the lands within Poland that they had been granted by Casimir III, both were asked to pay huge reparations. The effort to pay this essentially bankrupted the Knights and led to considerable unrest within their territory. John meanwhile could rely on Gothenland's slightly more robust treasury to pay his share and although he did impose extra taxes in 1378/79 there was considerably less upheaval in Gothenland's pockets of territory in Pomerelia and Samland compared to the large Prussian territory held by the Knights. The victory silenced the remaining rebellious Polish nobles.

In August 1382 John I died, nominally leaving his kingdoms to his son-in-law Wenceslaus I of Luxembourg. The Polish sejm had effectively agreed to this following Torun, but in the event refused to be ruled from Prague or Buda. They would rally around John I's other daughter, Jadwiga of Poland, and soon had a league of anti-Luxembourg states, including Gothenland, backing their stance. Wenceslaus' Bohemian army would be heavily defeated at Jezierzyca River, with Wenceslaus barely escaping capture. As a reward for Gothenland's assistance, John was granted tax rights over Pomerelia, as well as full lordship over several towns along the coast, for which he paid homage.

In practice Jadwiga left her Lithuanian husband Jogaila (Wladyslaw II) as effective ruler of Poland and he was much less-disposed to Gothenland and the Knights encroaching on Polish territory. John however was savvy enough to ally himself to whichever causes Wladyslaw pursued, whereas the Knights (their raison d'etre now virtually gone) supported a succession of Lithuanian nobles vying for control. In 1390 when Wladsylaw's cousin Vytautas began a civil war for the Grand Duchy with suport from the Teutonic Knights, the Gothenlandic armies, with John at its head, besieged Elblings and Framborg, and won a small but notable victory over the Knights at Wormditt. Seeing the Knights as a necessary evil to balance Polish and Lithuanian politics Wladyslaw did not punish them (or reward Gothenland) to the degree which may have been expected. John was rebuffed when he asked for Elblings and Königsberg (which have considerably consolidated his Samland territory plus removed the friction between the German Bishop of Samland and the Gothenlandic/Prussian priests in Gothenland's Samland). John was however given express rights over the Pomerelian coastline from Slupsk to the Vistula, though this excluded Gdansk which still remained in the Knights hands.

John would die in 1403 and was succeeded by his eldest son Eric. Between 1403 and 1416 all three of John's surviving children would occupy the throne, a situation which would lead to a breakdown in law and order and civil war, the so-called 'Herring-Hook War', between factions loyal to John's grandson Christopher or his niece Magdalena.