Index out of range exception

I realise this is probably a pretty common question, but I've looked at other answers and I really have no idea why these errors are coming up.

I have dragged four sounds into inspector to set up the array, and when I play the scene the ladder sounds work perfectly fine, as in it randomly chooses one of the four sounds and plays them like it should.

The error:

 IndexOutOfRangeException: Array index is out of range.
 WalkSoundScript+$PlayLadderSound$160+$.MoveNext() (at Assets/scripts /WalkSoundScript.js:83)

The Script in question:

 #pragma strict
 
 var sounds : AudioClip[];
 var ladderSounds : AudioClip[];
 
 var sneakSpeed : float = 0.04;
 var runSpeed : float = 0.06;
 var sneakDelay : float = 0.8;
 var runDelay : float = 0.35;
 var sneakVolume : float = 0.5;
 var runVolume : float = 1.0;
 var ladderDelay : float = 0.8;
 var ladderVolume : float = 0.2;
 var isGuard : boolean = false;
 
 private var lastPosition : Vector3 = Vector3.zero;
 private var isPlaying : boolean = false;
 private var isPlayingLadder : boolean = false;
 private var isJumping : boolean = false;
 
 private var speed : float = 0f;
 
 function Start () {
 
 }
 
 function Update () {
     if(speed > runSpeed){
         PlaySound(runDelay, runVolume);
     }
     else if(speed > sneakSpeed){
         PlaySound(sneakDelay, sneakVolume);
     }
     
     if(PlayerScript.onLadder && Mathf.Abs(Input.GetAxis("Vertical")) != 0 && !isGuard){
         PlayLadderSound();
     }
 }
  
 function FixedUpdate(){
     speed = (transform.position - lastPosition).magnitude;
     lastPosition = transform.position;
 }
 
 function PlaySound(delay : float, volume : float){
     if(isGuard){
         if(!isPlaying){
             audio.Stop();
             audio.volume = volume * 0.75;
             var num = Random.Range(0,3);
             audio.clip = sounds[num];
             isPlaying = true;
             audio.Play();
             
             yield WaitForSeconds(delay);
             isPlaying = false;
         }
     }else if(!isPlaying && PlayerAnimScript.grounded && !PlayerScript.onLadder){
         audio.Stop();
         audio.volume = volume;
         num = Random.Range(0,3);
         audio.clip = sounds[num];
         isPlaying = true;
         audio.Play();
         
         yield WaitForSeconds(delay);
         isPlaying = false;
     }
 }
 
 function PlayLadderSound(){
     if(!isPlayingLadder){
         
         audio.Stop();
         audio.volume = ladderVolume;
         var num = Random.Range(0,4);
         Debug.Log(num);
         audio.clip = ladderSounds[num];
         isPlayingLadder = true;
         audio.Play();
         
         yield WaitForSeconds(ladderDelay);
         isPlayingLadder = false;
     }
 }
 
 function PlayJump(){
     if(PlayerAnimScript.grounded && !isJumping){
             isJumping = true;
             AudioSource.PlayClipAtPoint(sounds[3], transform.position);
         
             yield WaitForSeconds(0.3);
             isJumping = false;
     }
 }