Wikipedia:Selected anniversaries/August 4
- 1578 - King Sebastian I (pictured) disappeared in the Battle of Alcácer Quibir, leading to a dynastic crisis in Portugal.
- 1790 - A newly passed tariff act in the United States established the Revenue Cutter Service.
- 1914 - World War I: Adhering to the terms in the 1839 Treaty of London, the United Kingdom declared war on Germany in response to the German invasion of Belgium.
- 1964 - Gulf of Tonkin Incident: Two destroyers of the U.S. Navy were reportedly attacked by North Vietnamese forces in the Gulf of Tonkin.
- 1983 - A coup d'état led by Blaise Compaoré made Thomas Sankara President of Upper Volta.