There are several possible causes of numbness in the 4th finger of the left hand: 1. cervical spondylosis, when cervical spondylosis has a herniated disc or osteophytes, it can stimulate and compress the nerve roots and cause numbness in the fingers. When there is chronic inflammation or tumor compression around the nerve sulcus, as well as scar adhesions of trauma, the ulnar nerve can be compressed, resulting in the symptoms of numbness of the 4th finger.