The 4th Army was a Royal Yugoslav Army formation mobilised prior to the German-led Axis invasion of the Kingdom of Yugoslavia during World War II. It was drawn from the peacetime 4th Army District. When mobilised, it consisted of three divisions, a brigade-strength detachment, one horse cavalry regiment and one independent infantry regiment. It formed part of the 1st Army Group, and was responsible for defending a large section of the Yugoslav–Hungarian border, being deployed behind the Drava river between Varaždin and Slatina. Like all Yugoslav formations at the time, the 4th Army had serious deficiencies in both mobility and firepower.

The Yugoslav Army, commonly the Royal Yugoslav Army, was the land warfare military service branch of the Kingdom of Yugoslavia. It existed from the Kingdom's formation in December 1918, until its surrender to the Axis powers on 17 April 1941. Aside from fighting along the Austrian border in 1919 and 1920 related to territorial disputes, and some border skirmishes on its southern borders in the 1920s, the JV was not involved in fighting until April 1941 when it was quickly overcome by the German-led invasion of Yugoslavia.