The sacral spinal nerve 4 (S4) is a spinal nerve

Spinal nerve

The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body...

 of the sacral segment..

It originates from the spinal column from below the 4th body of the sacrum

Sacrum

In vertebrate anatomy the sacrum is a large, triangular bone at the base of the spine and at the upper and back part of the pelvic cavity, where it is inserted like a wedge between the two hip bones. Its upper part connects with the last lumbar vertebra, and bottom part with the coccyx...

 

Muscles

S4 supplies many muscles, either directly or through nerves originating from S4. They are not innervated with S4 as single origin, but partly by S4 and partly by other spinal nerves. The muscles are:

• iliococcygeus
  Iliococcygeus muscle
  The Iliococcygeus arises from the inner side of the ischium and from the posterior part of the tendinous arch of the obturator fascia, and is attached to the coccyx and anococcygeal raphé; it is usually thin, and may fail entirely, or be largely replaced by fibrous tissue...
  
  
• puborectalis
  Puborectalis muscle
  The fibers which form a sling for the rectum are named the Puborectalis or Sphincter recti.They arise from the lower part of the pubic symphysis, and from the superior fascia of the urogenital diaphragm....
  
  
• coccygeus
  Coccygeus muscle
  The Coccygeus is a muscle of the pelvic wall , located posterior to levator ani and anterior to the sacrospinous ligament....
  
  
• Sphincter ani externus muscle
  Sphincter ani externus muscle
  The Sphincter ani externus is a flat plane of muscular fibers, elliptical in shape and intimately adherent to the integument surrounding the margin of the anus.-Anatomy:...
  
  
• sphincter urethrae membranaceae