Toggle through sprites on UI button press

Hi,

I've been fiddling around with this issue for a while now, I am having a problem toggling through four sprites when pushing a 'next' button. I can't change the scene because there is a video playing next to the text. I'm not seeing any errors thrown and I've filtered through tons of forums and tutorials, but I am still stuck. If anyone can point me in the direction of a tutorial or see anything wrong with the code, I would be very grateful.

Thank you so much in advance, the forums have really helped me learn a lot about this program.

 using UnityEngine;
 using System.Collections;
 using UnityEngine.UI;
 
 public class ButtonToggle : MonoBehaviour {
     
     public Sprite PanelOne;
     public Sprite PanelTwo;
     public Sprite PanelThree;
     public Sprite PanelFour;
     private SpriteRenderer spriteRender;
 
 
     void Start () {
         spriteRender = GetComponent<SpriteRenderer> ();
         /*if (spriteRender.sprite == null) {
             spriteRender.sprite = PanelOne;
         }*/
     }
 
     public void changeSprite () {
         if (spriteRender.sprite == PanelOne) {
             spriteRender.sprite = PanelTwo;
         } else if (spriteRender.sprite == PanelTwo) {
             spriteRender.sprite = PanelThree;
         } else if (spriteRender.sprite == PanelThree) {
             spriteRender.sprite = PanelFour;
         }
          else  {
             spriteRender.sprite = PanelFour;
         }
     }
 }