Two X-4s were built by the Northrop Corporation, but the first was found to be mechanically unsound and after 10 flights it was grounded and used to provide parts for the second.

• 诺斯罗普公司共生产了两架X-4，但是第一架被发现有一些机械上的问题，于是这架飞机在完成了10次飞行后被拆解。拆解后的飞机被用于为第二架X-4提供备用零件。