I have l3-4 spinal stenosis and left para central herniation. would pressure from herniation affect sphincter muscle and cause minor bowel incontinence?

Yes: Yes and if you have incontinence surgery should be considered.

Cauda equina syndrom: There is a condition called cauda equina syndrome in which very severe compression of the lumbar nerves can result in bowel/bladder incontinence - however this condition is rare. It sounds like you have already had an MRI based on the wording of your question, so the MRI will show whether or not you do have very severe compression of the lumbar nerves - talk to your physician about this.

L3-4 disk and bowels: Generally, spinal canal compromise of significant severity, called cauda equina syndrome, can cause bladder and bowel dysfunction, but usually overflow incontinence of bladder and constipation.

Lumbar Radicilopathy: Disc herniation has to cause severe compression of the nerve roots( cauda equina ) to result in bowel and bladder problems. Usually it will cause back or leg pain, .