Platelets 4×10⁹/L belongs to very severe thrombocytopenia and the condition is serious. Platelets are small pieces of cytoplasm shed from the cytoplasmic cleavage of mature megakaryocytes of bone marrow, which are extremely important for hemostatic function. After trauma occurs, platelets can quickly adhere to the trauma site and gather into clusters to form hemostatic emboli for hemostasis. Obvious bleeding symptoms can occur when platelet count decreases, manifesting as bleeding spots on the skin and mucous membranes, petechiae, vomiting blood, black stools, and so on. Many causes can cause thrombocytopenia, such as aplastic anemia, autoimmune thrombocytopenic purpura, leukemia and so on. Some tumor patients can also cause platelet drop after chemotherapy due to damage to hematopoietic stem cells. When platelets are 4×10⁹/L, they are very likely to be life-threatening due to internal bleeding such as cerebral hemorrhage, or bleeding out after trauma, which is very serious and needs to receive emergency treatment. After the discovery of platelet decrease, you should actively improve the examination to clarify the cause, and standardize the treatment under the guidance of the doctor.