sacral spinal nerve 4 sacral spinal nerve 4 the sacral spinal nerve 4 (s4) is a spinal nerve of the sacral segment.sacral spinal nervethe plan of the lumbosacral plexusdetailsidentifierslatinnervi spinalisfma6426anatomical terminology it originates from the spinal column from below the 4th body of the sacrum sacrum, showing bodies in center. 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 puborectalis coccygeus sphincter ani externus muscle sphincter urethrae membranaceae additional images sacral spinal nerve 4