Disable function OnGUI after 4 seconds

I wrote a script that should disable after 4 seconds the OnGUI function, but when I tried, the function OnGUI still working after 4 seconds.

This is my JavaScript:

 #pragma strict
 var x : boolean = true;
 
 function Start () {
 yield WaitForSeconds(4);
 x = false;
 }
 
 function Update () {
 if(x == true){
 OnGUI();
 }
 }
 
 function OnGUI(){
 GUI.Label(new Rect(Screen.width/2 - 75, Screen.height - 100, 400, 30), "What's that noise? It's was form the living room...");
 }

Can anyone help me? Thanks!