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.