The Type 4 20 cm rocket mortar (四式二十糎噴進砲, Yonshiki nijū-senchi funshinhō) was a 203 mm rocket mortar used by the Imperial Japanese Army in the final stages of World War II.

[2] The Type 4 20cm rocket mortar was developed in the final stages of World War II by the Japanese Army Technical Bureau, as a low-cost, easy to produce weapon, which had an advantage of greater accuracy over conventional mortars.

[2] The fuzes were the same used on the Type 100 mortar round and could be either be set for instantaneous or time-delay action.

Due to its ease of construction and portability, the Type 4 was produced in large numbers and distributed to hidden arsenals for use as last-ditch weapons during the projected Allied invasion of the Japanese home islands.

The U.S. Army intelligence assessment of the system stated that the Type 4 20cm rocket mortar was a “simple, sturdy, mobile device, capable of fine adjustment for accurate fire.”[1] Although the weapon came with a standard mortar tube with tripod mounting, if necessary, the rocket-propelled round could be launched from an ordinary pipe or culvert with sufficient diameter, wooden rails, or even directly from a slope in the ground.