Moving a GameObject on a square patern

My goal is to move a GameObject on a square patern, in order to do it I made this script: public class SquarePatern : MonoBehaviour {

     public Transform target;
     public Transform target1;
     public Transform target2;
     public Transform target3;
     public Transform target4;
     public float speed;
     void Update()
     {
         float step = speed * Time.deltaTime; 
         transform.position = Vector3.MoveTowards(transform.position, target.position, step);
     }
     void OnCollisionEnter(Collision col)
     {
         if (col.gameObject.name == "Target1")
         {
             target = target2;
         }
         if (col.gameObject.name == "Target2")
         {
             target = target3;
         }
         if (col.gameObject.name == "Target3")
         {
             target = target4;
         }
         if (col.gameObject.name == "Target4")
         {
             target = target1;
         }
     }
 }

I ensured that the 4 transform have a box Collider and have the same name as in the script, but somehow it just doesn't work, the GameObject goes to the target1 and stop on it, what did i do wrong?