The 4th Naval Armaments Supplement Program (マル4計画, 第四次海軍軍備充実計画, Maru 4 Keikaku, Dai-Yo-Ji Kaigun Gunbi Jūjitsu Keikaku) was one of the armaments expansion plan of the Imperial Japanese Navy (IJN).
Background
In 1939, the IJN started new naval armaments expansion plan. It extended to 80 warships and 75 Naval Air Groups by 1.6 billion JPY.
References
4th Naval Armaments Supplement Programme Wikipedia(Text) CC BY-SA