Help with college assignment

Ok so for a college assignment I'm making a game with a carnival theme, its for a education game. Its going to be a set of mini-games and one of the mini-games is that when you go up to a stall a game will start that asks you a maths question and their will be 4 boxes on a shelf and you have to pick the right answer that will be on them.

Now what I'm looking for is that the boxes with the answers don't stay in the same position all the time, that they swap around with each other.

So pretty much just to make sure people know what I mean here would be the stall:

1234 I want it that so everytime I load the scene up their in a different position such as 3142

I have this code:

var timer : float = 0.0;

var spawning : boolean = false;

var prefab : Rigidbody;

var spawn1 : Transform;

var spawn2 : Transform;

var spawn3 : Transform;

var spawn4 : Transform;

function Update () {

//check if spawning at the moment, if not add to timer

if(!spawning){

timer += Time.deltaTime;

}

//when timer reaches 2 seconds, call Spawn function

if(timer >= 0.1){

Spawn();

}

}

function Spawn(){

//set spawning to true, to stop timer counting in the Update function

spawning = true;

//reset the timer to 0 so process can start over

timer = 0;

//select a random number, inside a maths function absolute command to ensure it is a whole number

var randomPick : int = Mathf.Abs(Random.Range(1,5));

//create a location 'Transform' type variable to store one of 3 possible locations declared at top of script

var location : Transform;

//check what randomPick is, and select one of the 3 locations, based on that number

if(randomPick == 1){

location = spawn1;

Debug.Log("Chose pos 1");

}

else if(randomPick == 2){

location = spawn2;

Debug.Log("Chose pos 2");

}

else if(randomPick == 3){

location = spawn3;

Debug.Log("Chose pos 3");

}

else if(randomPick == 4){

location = spawn3;

Debug.Log("Chose pos 4");

}

//create the object at point of the location variable

var thingToMake : Rigidbody = Instantiate(prefab, location.position, location.rotation);

}

Which does work that the boxes show up in random positions on the stall but the only thing is that they sometimes spawn at the same spawn position and therefore hit of each other, can anyone help with code that will check if one of the boxes is already their and therefore spawn in another one of the spawn points.

Thanks.