My 4y.O has freq neck pain-noticed most on days she has pt where they work on heel-cord tightness. Her MRI ruled out tethered cord. Could it be wrong?

Very unlikely. But have the doctors assessed her upper spine. Most of these kids have a natural level of inflexibility which hurts when overuse of the muscles occurs, such as during therapy. A rare condition such as a vertebral anomaly or a chiari malformation may cause such symptoms. Unfortunately, space limitations prevents an explanation, but ask your pediatrician.
Symptom management. Assuming your child is not currently ill with fever or sore throat, and/ or there has been no recent history of trauma, most neck pain in children this age is due to a muscle spasm called torticollis. Massage, plus stretching exercises may help relieve head and neck pain. A tethered cord has other symptoms that may include gait abnormalities and bowel and bladder dysfunction.