High Quality Content by WIKIPEDIA articles! The sacral spinal nerve 4 (S4) is a spinal nerve of the sacral segment. The term spinal nerve generally refers to the mixed spinal nerve, which is formed from the dorsal and ventral roots that come out of the spinal cord. A spinal nerve emerges at two points from the spinal cord, the ventral and dorsal roots. The ventral and dorsal roots merge to form the whole spinal nerve. The spinal nerve emerges from the spinal column through an opening (intervertebral foramen) between adjacent vertebrae.