John IV Doukas Laskaris (or Ducas Lascaris) (Greek: Ἰωάννης Δ΄ Δούκας Λάσκαρις) was emperor of Nicaea from August 18, 1258 to December 25, 1261. This empire was one of the Greek states formed from the remaining fragments of the Byzantine Empire, after the capture of Constantinople by Roman Catholics during the Fourth Crusade in 1204. It lasted until 1261, when the Nicaeans restored the Byzantine Empire in Constantinople.
John was a son of Theodore II Doukas Laskaris and Elena of Bulgaria. His maternal grandparents were Emperor Ivan Asen II of Bulgaria and his second wife Anna Maria of Hungary. Anna was originally named Mária and was the eldest daughter of Andrew II of Hungary and Gertrude of Merania.
John IV was only seven years old when he inherited the throne on the death of his father. The young monarch was the last member of the Laskarid dynasty, which had done much to restore the Byzantine Empire. His regent was originally the bureaucrat George Mouzalon, but Mouzalon was murdered by the nobility, and the nobles' leader Michael Palaiologos usurped the post. Soon, on January 1, 1259, Palaiologos made himself co-emperor as Michael VIII.
After Michael's conquest of Constantinople from the Latin Empire on July 25, 1261, John IV was left behind at Nicaea, and was later blinded on Michael's orders on his eleventh birthday, December 25, 1261. This made him ineligible for the throne, and he was exiled and imprisoned in a fortress in Bithynia. This action led to the excommunication of Michael VIII Palaiologos by the Patriarch Arsenius Autoreianus, and a later revolt led by a Pseudo-John IV near Nicaea.
John IV spent the remainder of his life as monk in Dacibyza, under the name Joasaph.