| 4 April 850 | Carolingian Empire, Italy [political events] | Louis II, son of the Frankish emperor Lothair I, is crowned in Rome as emperor and king of Italy. |
| 4 April 871 | Wessex [political events] | Alfred the Great becomes king of Wessex, England, on the death of his brother, King Aethelred. |
| 4 April 1081 | Poland [revolution] | A rebellion in Poland forces King Boleslaw II into exile. He is succeeded by his brother, Wladyslaw I Herman, under whom the Polish territories disintegrate in civil war, and the crown falls into abeyance until 1300. |
| 4 April 1140 | Saxony, Holy Roman Empire, Germany [political events] | The Saxons reject Albert I the Bear as their duke and refuse to surrender to Conrad III Hohenstaufen, king of the Germans. . |
| 4 April 1147 | Almohad Emirate, Spain, Almoravid Emirate [Reconquista of Spain (711–1492)] | The Almohad Mahdi (Muslim leader) `Abd-al-Mu'min completes his conquest of the Almoravid possessions in Morocco with the capture of the capital city of Marrakesh; he then crosses into Spain, where the Almoravid emirate has disintegrated into several kingdoms established in Córdoba, Valencia, Murcia, and other areas. |
| 4 April 1230 | Bulgaria, Byzantine Empire [wars] | John Asen II, Tsar of Bulgaria, defeats and captures Theodore Angelus, the despot of Epirus, at Klokotinitza; the Bulgarian Empire now extends from the Black Sea to the River Danube, the Adriatic Sea, and Thessaly. |
| 4 April 1231 | Papal States, Holy Roman Empire [human rights] | Pope Gregory IX sets up the Holy Office (Inquisition), a permanent tribunal for investigating the various heresies proliferating in France and Germany. |
| 4 April 1233 | Cyprus, Holy Roman Empire, Kingdom of Jerusalem [wars] | With the surrender of the Cypriot port of Kyrenia to John of Ibelin, Lord of Beirut, the Holy Roman Emperor Frederick II's forces are expelled from Cyprus and Henry of Lusignan is restored as king. |
| 4 April 1248 | Mongol Empire [political events] | The death of Guyuk, Great Khan of the Mongols, is followed by an interregnum which lasts until 1251. |
| 4 April 1259 | Holy Roman Empire [political events] | Pope Alexander IV recognizes Richard, Earl of Cornwall, as King of the Romans. |
| 4 April 1268 | Nicaean Empire, Venice, Genoa [treaties] | The Nicaean emperor Michael VIII Palaeologus makes peace with the Venetians and restores their trading privileges, though at the same time keeping a similar treaty with Genoa. This double treaty gives distinct political and trading advantages to the Nicaeans. |
| 4 April 1305 | Byzantine Empire, Balkans [political events] | Roger de Flor, the captain of the Catalan mercenary Grand Company, is murdered in Constantinople. His followers defeat the imperial troops of Andronicus II Palaeologus, the Byzantine emperor, and plunder Thrace. |
| 4 April 1346 | Italy, Holy Roman Empire, Hungary [political events] | By abandoning imperial claims in Italy, Charles of Bohemia receives the consent of Pope Clement VI to his election as king of the Romans. Pope Clement VI subsequently orders the German electors to choose a king to replace Ludwig I of Hungary. |
| 4 April 1505 | Holy Roman Empire, Hungary, Bavaria, Germany [political events] | At the Diet (legislative assembly) of Cologne, the Holy Roman Emperor Maximilian I, strengthened by his victory in the Landshut War over the duchy of Bavaria-Landshut, obtains support from the German princes for an expedition to Hungary to assist King Ladislas II against his factious nobility; constitutional reforms are designed to further promote his aim of a universal Habsburg monarchy. Bavaria-Landshut is divided between the claimants. |
| 4 April 1507 | Holy Roman Empire, Spanish Netherlands [political events] | The States-General (parliament) of the Netherlands appoints a regency council for the minority of Archduke Charles (the future Holy Roman Emperor Charles V). It is effectively headed by his aunt Margaret of Austria. |
| 4 April 1525 | Holy Roman Empire [political events] | Even as it spreads to Alsace and northern Swiss subject territories, the German Bauernkrieg (‘Peasants' War’) revolution experiences its first defeat, by the army of the Swabian League (Habsburg allies) under the Truchesse von Waldburg, at Leipheim, on the River Danube. |
| 4 April 1552 | Holy Roman Empire, France [wars] | Maurice, Elector of Saxony, and Albert Alcibiades II, Margrave of Brandenburg-Kulmbach, leading the German Protestant armies, in concert with King Henry II of France in Lorraine, against the Holy Roman Emperor Charles V, take Augsburg, the largest and richest German city. |
| 4 April 1588 | Denmark-Norway [political events] | Christian IV accedes to the throne of Denmark-Norway on the death of King Frederick II. |
| 4 April 1611 | Denmark-Norway, Sweden [wars] | King Christian IV of Denmark forces the Riksråd (parliament) to declare war on Sweden (the War of Kalmar), hoping to reconquer the country. Danish forces take the Swedish port of Kalmar and the mouth of the Göta River the following month. |
| 4 April 1617 | Scotland [births and deaths] | John Napier (or Neper), Scottish mathematician and theologian who developed the concept of logarithms, dies in Merchiston Castle, near Edinburgh, Scotland (67). |
| 4 April 1841 | USA [administration] | Following the death of William H Harrison after just one month in office, John Tyler becomes the tenth president of the USA. |
| 4 April 1873 | UK, Africa [wars] | War breaks out between Britain and the Ashanti in west Africa (modern Ghana) as a result of British attempts to stop King Kofi Kari-Kari's slave trade. |
| 4 April 1891 | Germany [colonization] | The Pan-German League is founded, a popular association dedicated to agitating for German expansionism. |
| 4 April 1904 | Australia [political parties] | John Christian Watson becomes the world's first Labour prime minister, in Australia. |
| 4 April–1 May 1919 | Germany [law and government] | A soviet republic is established in Bavaria, Germany, by communists, following a radicalization of politics in the wake of the assassination of Kurt Eisner. |
| 4 April 1949 | USA, Canada, UK, France, Luxembourg, Belgium, Netherlands, Italy, Portugal, Denmark, Iceland, Norway [international organizations] | The North Atlantic Treaty Organization (NATO) is founded to provide mutual support against the Soviet military presence in eastern Europe. The treaty is signed by the USA, Canada, Britain, France, Luxembourg, Belgium, the Netherlands, Italy, Portugal, Denmark, Iceland, and Norway. |
| 4 April–30 May 1959 | French West Africa [diplomacy] | The autonomous French West Africa colonies of Ivory Coast, Niger, Upper Volta (now Burkina Faso), and Dahomey (now Benin) sign a series of agreements to form the Sahel–Benin Union. |
| 4 April 1968 | USA [political events] | The assassination of Black American civil-rights activist Martin Luther King, Jr, in Memphis, Tennessee, sparks a week of rioting in black ghettos throughout the nation. His assassin, James Earl Ray, is arrested in London, England, on 8 June and promptly extradited to the USA. |
| 4 April 1968 | USA [births and deaths] | Martin Luther King, Jr, US Baptist minister and civil-rights leader, is assassinated in Memphis, Tennessee, by a sniper later identified as escaped convict James Earl Ray (39). |
| 4 April 1981 | UK [sports] | Susan Brown becomes the first woman cox in the Oxford and Cambridge university boat race in England, steering the Oxford crew to victory. |
| 4 April 1984 | UK [political events] | Bailiffs are brought in to clear the women's peace camp at Greenham Common, England, the site of a NATO base for cruise missiles. |
| 4 April 1991 | USA [ecology] | The US Environmental Protection Agency announces ozone layer depletion at twice the speed previously predicted. |
| 4 April 2006 | USA [physiology] | Successful bladder transplants into seven young patients, with organs engineered using live tissue in the laboratory, are announced by a team of US research scientists in Boston. The medical breakthrough could herald the regeneration of hearts and other human organs in the future. |
