The positive significance of the 4-letter test has two main aspects: 1. It can help diagnose sacroiliac arthritis or ankylosing spondylitis leading to lesions in the sacroiliac joint. When the 4-letter test is performed, because the joint surface of the sacroiliac joint position will be stretched and stimulated, if there is a lesion, it will lead to greater friction in the joint cartilage area and a significant pain, so that it can diagnose sacroiliac arthritis. 2. Femoral head necrosis and or hip joint with lesions, because when doing the 4 tests, the joint capsule between the femoral head of the hip joint and the acetabulum or the joint, as well as the surrounding soft tissues will be strained and stimulated by the pressure, it will be easy to have obvious pain. Therefore, in clinical practice, if there is a significant pain in the hip joint of the thigh during the 4-test, it can be judged that the hip joint has a lesion or femoral head necrosis.