The S4 vertebra, also known as the fourth sacral vertebra, is one of the five fused vertebrae that make up the sacrum in the human spine. The sacrum is located at the base of the spine, between the two pelvic bones, and forms part of the pelvis.
Located below the S3 vertebra, S4 is shaped like a triangular bone and is larger than the uppermost sacral vertebra (S1). S4 nerves primarily affect the perineal area (the region between the anus and external genitalia).
