It is a normal phenomenon for a four-month-old baby to have rattling of the bones when moving around. As the child is relatively young, the ligaments are weak, the joint sockets are shallow, the ligaments around the joints are lax, the bones are soft, the surrounding muscle strength is weak, and there are cartilage plates at the epiphyses of the long bones, it is possible that popping sounds may occur when the main joints do flexion and extension activities. As the child gets older, the ligaments gradually become stronger and the muscle strength gradually increases, the popping sound will disappear. If your child has hip snapping, you need to consider whether there are congenital abnormalities in hip development, such as dislocation or dysplasia, and you need to go to the hospital for an ultrasound examination to determine the angle of the hip joint. Especially for children under 6 months old, ultrasound examination can diagnose, if there is an abnormality, you need to give the child a brace or sling treatment, the earlier the treatment of this kind of disease, the better the effect.