Louis IV (German: Ludwig) (1 April 1282 – 11 October 1347), called the Bavarian, of the house of Wittelsbach, was the King of Germany (King of the Romans) from 1314, the King of Italy from 1327 and the Holy Roman Emperor from 1328.

Louis IV was Duke of Upper Bavaria from 1294 /1301 together with his elder brother Rudolf I, served as Margrave of Brandenburg until 1323 and as Count Palatine of the Rhine until 1329, became also Duke of Lower Bavaria in 1340 and Count of Hainaut, Holland, Zeeland and Friesland in 1345.

Louis was born in Munich, the son of Louis II, Duke of Upper Bavaria and Count Palatine of the Rhine, and Matilda, a daughter of King Rudolph I. Though Louis was partly educated in Vienna and became co-regent of his brother Rudolf I in Upper Bavaria in 1301 with the support of his Habsburg mother Matilda and her brother King Albert I, he quarrelled with the Habsburgs from 1307 over possessions in Lower Bavaria. A civil war against his brother Rudolf due to new disputes on the partition of their lands was ended in 1313, when peace was made at Munich. In the same year Louis defeated his Habsburg cousin Frederick the Fair. Originally, he was a friend of Frederick, with whom he had been raised. However, armed conflict arose when the guardianship over the young Dukes of Lower Bavaria (Henry XIV, Otto IV and Henry XV) was entrusted to Frederick. On 9 November 1313, Frederick was beaten by Louis in the Battle of Gamelsdorf and had to renounce the tutelage.

After the death of Holy Roman Emperor Henry VII, the Luxemburg party among the prince electors set aside Henry's son, the Bohemian king John of Luxemburg, because of his youth and chose Louis as rival king to Frederick the Fair, the cousin of Louis. Louis was elected in October 1314 upon the instigation of Peter of Aspelt, the Prince-elector and Archbishop of Mainz, with five of the seven votes, to wit Archbishop-Elector Baldwin of Trier, the legitimate King-Elector John of Bohemia, Duke John II of Saxe-Lauenburg, rivallingly claiming the Saxon prince-electoral power, Peter of Aspelt, and Prince-Elector Waldemar of Brandenburg. Frederick the Fair received in the same election four of the seven votes, with the deposed King-Elector Henry of Bohemia, illegitimately assuming electoral power, Archbishop-Elector Henry II of Cologne, Louis's own brother Prince-Elector Rudolph Iof the Electoral Palatinate, and Duke Rudolph I of Saxe-Wittenberg, rivallingly claiming the Saxon prince-electoral power.

Louis then was quickly crowned in Aachen by Peter of Aspelt, while Frederick was crowned in Bonn by Prince-Elector Henry II of Cologne. In the following conflict between both kings Louis recognized in 1316 the independence of Switzerland from Habsburg. After several years of bloody war, victory finally seemed within the grasp of Frederick, who was strongly supported by his brother Leopold. However, Frederick's army was in the end decisively beaten in the Battle of Mühldorf on 28 September 1322 on the Ampfing Heath, where Frederick and 1300 nobles from Austria and Salzburg were captured.

Louis held Frederick captive in Trausnitz Castle (Schwandorf) for three years, but the determined resistance by Frederick's brother Leopold, the retreat of the King of Bohemia John of Luxembourg from his alliance, and the Pope's ban induced Louis to release Frederick in the Treaty of Trausnitz of 13 March 1325. In this agreement, Frederick finally recognized Louis as legitimate ruler and undertook to return to captivity if he did not succeed in convincing his brothers to submit to Louis. As he did not manage to overcome Leopold's obstinacy, Frederick returned to Munich as a prisoner, even though the Pope had released him from his oath. Louis, who was impressed by such nobility, renewed the old friendship with Frederick and they both agreed to rule the Empire jointly.

Since the Pope and the electors strongly objected to this agreement, another treaty was signed at Ulm on 7 January 1326, according to which Frederick would administer Germany as King of the Romans, while Louis would be crowned as Holy Roman Emperor in Italy. However, after Leopold's death in 1326, Frederick withdrew from the regency of the Empire and returned to rule only Austria. He died on 13 January 1330.

Despite Louis' victory, Pope John XXII still refused to ratify his election, and in 1324 he excommunicated Louis, but the sanction had less effect than in earlier disputes between emperors and the papacy.

After the reconciliation with Habsburg in 1326, Louis marched to Italy and was crowned King of Italy in Milan in 1327. Already in 1323 Louis had sent an army to Italy to protect Milan against the Kingdom of Naples which was together with France the strongest ally of the papacy. But now the Lord of Milan Galeazzo I Visconti was disposed since he was suspected of conspiring with the pope.

In January 1328 Louis entered Rome and had himself crowned emperor by the aged senator Sciarra Colonna, called captain of the Roman people. Three months later Louis published a decree declaring "Jacque de Cahors" (Pope John XXII) deposed on grounds of heresy. He then installed a Spiritual Franciscan, Pietro Rainalducci as Antipope Nicholas V, who was deposed after Louis had left Rome in early 1329. In fulfilment of an oath, on his return from Italy Louis founded Ettal Abbey on 28 April 1330. Philosophers such as Michael of Cesena, Marsilius of Padua and William of Ockham who advocated a form of church/state separation were now protected at the emperor's court in Munich.

The failure of later negotiations with the papacy led in 1338 to the declaration at Rhense by six electors to the effect that election by all or the majority of the electors automatically conferred the royal title and rule over the empire, without papal confirmation.

Louis also allied in 1337 with Edward III of England against Philip VI of France, the protector of the new Pope Benedict XII in Avignon. Philip had prevented any agreement between the emperor and the pope. In 1338 Edward III was the emperor's guest at the Imperial Diet in the Kastorkirche at Coblence and was named vicar-general of the Holy Roman Empire. In 1341 Louis deserted Edward but came only temporarily to terms with Philip. The expected English payments were missing and Louis intended to reach an agreement with the pope one more time.

Louis IV was a protector of the Teutonic Knights. In 1337 he allegedly bestowed upon the Teutonic Order a privilege to conquer Lithuania and Russia, although the Order had only petitioned for three small territories. Later he forbade the Order to stand trial before foreign courts in their territorial conflicts with foreign rulers.

Louis concentrated his energies also on the economic development of the cities of the empire, so his name can be found in many city chronicles for the privileges he granted. In 1330 the emperor permitted the Frankfurt Trade Fair and Lübeck as the most powerful member of the Hanseatic League received in 1340 as first city of the empire the coinage prerogative for golden gulden.

In 1323 Louis gave Brandenburg as a fiefdom to his eldest son Louis V. With the Treaty of Pavia the emperor returned the Palatinate to his nephews Rudolf and Rupert in 1329. The duchy of Carinthia was released as an imperial fief on 2 May 1335 in Linz to his Habsburg relatives Albert II, Duke of Austria and Otto, Duke of Austria.

With the death of duke John I in 1340 Louis inherited Lower Bavaria and then reunited the duchy of Bavaria. John's mother, a member of the Luxemburg dynasty, had to return to Bohemia. In 1342 Louis also acquired Tyrol for the Wittelsbach by voiding the first marriage of Margarete Maultasch with John Henry of Bohemia and marrying her to his own son Louis V, thus alienating the house of Luxemburg even more.

In 1345 the emperor further antagonized the lay princes by conferring Hainaut, Holland, Zeeland and Friesland upon his wife Margaret of Holland. The hereditary titles of Magaret's sisters, one of them was the queen of England, were ignored. Due to the dangerous hostility of the Luxemburg Louis had increased his power base ruthlessly.

The acquisition of these territories and his restless foreign policy had earned Louis many enemies among the German princes. In the summer of 1346 the Luxemburg Charles IV was elected rival king, with the support of Pope Clement VI. Louis himself obtained much support from the Imperial Free Cities and the knighthood and successfully resisted Charles, who was widely regarded as a papal puppet ("rex clericorum" as William of Ockham called him). Also the Habsburg dukes stayed loyal to Louis. In the Battle of Crécy Charles' father John of Luxemburg was killed; Charles himself also took part in the battle but escaped.

Louis died in October 1347 from a stroke suffered during a bear-hunt in Puch near Fürstenfeldbruck. He is buried in the Frauenkirche in Munich. Louis' sudden death avoided a longer civil war. The sons of Louis supported Günther von Schwarzburg as new rival king to Charles but finally joined the Luxemburg party after Günther's early death in 1349 and divided the Wittelsbach possessions among each other again. In continuance of the conflict of the House of Wittelsbach with the House of Luxemburg, the Wittelsbach family returned to power in the Holy Roman Empire in 1400 with King Rupert of Germany, a great-grandnephew of Louis.

In 1308 he married firstly to Beatrix of Świdnica. They had six children.
In 1324 he married secondly to Margaret of Holland, countess of Hainaut and Holland. They had ten children.