Problem with calling function calling order [C#]

Hi, i am having a problem in which i am trying to make a space invaders game, however when the cubes spawn they only descend when in a certain order, for example, thing 4 will only descend when thing 3 is descending, and thing 3 will only descend when thing 2 is descending, and thing 2 will only descend when thing 1 is descending. So they have to wait for thing 1 to spawn before the rest can descend etc. Thanks in advance, Matt.

 using UnityEngine;
 using System.Collections;
 
 public class moveUp : MonoBehaviour {
 
     public GameObject shot;
     public Transform pos1;
     public Transform pos2;
     public Transform pos3;
     public Transform pos4;
     public Rigidbody enemy;
     private Rigidbody thing1;
     private Rigidbody thing2;
     private Rigidbody thing3;
     private Rigidbody thing4;
     public float speed = 2f;
     private bool boolean = true;
     
     void Update(){
         StartCoroutine("hi");
         thing1.transform.Translate(Vector3.down * speed * Time.deltaTime);
         thing2.transform.Translate(Vector3.down * speed * Time.deltaTime);
         thing3.transform.Translate(Vector3.down * speed * Time.deltaTime);
         thing4.transform.Translate(Vector3.down * speed * Time.deltaTime);
 
     }
 
     public IEnumerator hi(){
 
         if(boolean == true){
 
             boolean = false;
 
             int random = Random.Range(1, 7);
         
             switch (random) {
             case 1:
                 thing1 = Instantiate(enemy, pos1.position, pos1.rotation) as Rigidbody;
                 break;
             case 2:
                 thing2 = Instantiate(enemy, pos2.position, pos2.rotation) as Rigidbody;
                 break;
             case 3:
                 thing3 = Instantiate(enemy, pos3.position, pos3.rotation) as Rigidbody;
                 break;
             case 4:
                 thing4 = Instantiate(enemy, pos4.position, pos4.rotation) as Rigidbody;
                 break;
             }
             yield return new WaitForSeconds(3);
 
             boolean = true;
 
         }
 
     }
 
 }