'Oboe IV' was the Allied unrealised plan for what was to have been the most important single element of the 'Oboe' series, namely the Australian recapture the Soerabaja area of Java island in the Japanese-occupied Netherlands East Indies by Lieutenant General Sir Leslie Morshead’s I Corps (May 1945).

Located on the north-east coast of Java, Soerabaja was the main port and Dutch naval base in the Netherlands East Indies in December 1941. The city was protected by 48 pieces of coastal artillery, but all were of relatively small calibre, and there were only eight 3.15-in (80-mm) guns for air defence. The facilities included at least one floating dry dock. The airfield at Perak possessed two 3,000-ft (915-m) runways and had facilities for both civil and military users, and the seaplane base at Morokrembangan, completed in July 1925, was one of the finest facilities of its type in the world, with 24 hangars, concrete ramps, underground fuel storage, and extensive repair, maintenance, and communication facilities. However, the harbour was somewhat shallow and could accommodate ships no larger than heavy cruisers despite continual dredging. The city’s population in 1941 was about 341,700 persons.

The area’s oilfields produced about 1 million barrels per year, and there was an important refinery at Wonokromo which produced a large amount of the lighter, more valuable fractions, such as petrol.

With the fall of the Netherlands to Germany in May 1940, the Dutch relocated all flight training to Java, and the Marine Luchts Dienst (naval air arm) re-established its flight school at Morokrembangan. The school was transferred once more, this time to Australia, in February 1942.

Soerabaja was attacked by Japanese aircraft (72 bombers and 44 fighters) on 3 February 1942, and was raided several more times after this until the city fell to the Japanese on 7 March 1942. Soerabaja and Java then remained under Japanese control for the rest of the war.

With air support from Banjarmasin secured in 'Oboe III', 'Oboe IV' was to have used Major General Jack E. S. Stevens’s 6^th Division, Major General Edward J. Milford’s 7^th Division, one infantry brigade of Major General George F. Wootten’s 9^th Division, and two tank battalions.

The campaign was to have started with the capture of Soerabaja on 27 June, followed by a westward advance to take Djakarta and Bandoeng, and an eastward advance to the Lombok Strait.