John IV Doukas Laskaris (or Ducas Lascaris) (Greek: Ἰωάννης Δ΄ Δούκας Λάσκαρις, Iōannēs IV Doukas Laskaris) (December 25, 1250 – c. 1305) 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.