This script is for changing 3 drop-downs but i want it to be 4. Someone that can help me with that?

Code:
<HTML>

<HEAD>

<TITLE>The JavaScript Source:  Forms:  Dropdown Box Population (Revised)</TITLE>

<META HTTP-EQUIV="The JavaScript Source" CONTENT = "no-cache">
<META NAME="description" CONTENT="Multiple series of dropdown boxes (up to 3) are automatically populated based on previous selection. Revised to work with IE, Netscape, Firefox, and Opera.">
<META NAME="date" CONTENT="2005-11-09">
<META NAME="channel" CONTENT="Developer">
<META NAME="author" CONTENT="DeWayne Whitaker">
<META NAME="section" CONTENT="Forms">

<script type="text/javascript">
<!--
/* This script and many more are available free online at
The JavaScript Source!! http://javascript.internet.com
Revised by: DeWayne Whitaker :: http://www.aecdfw.com
Original by: Andrew Berry */

var arrItems1 = new Array();
var arrItemsGrp1 = new Array();

arrItems1[3] = "Truck";
arrItemsGrp1[3] = 1;
arrItems1[4] = "Train";
arrItemsGrp1[4] = 1;
arrItems1[5] = "Car";
arrItemsGrp1[5] = 1;

arrItems1[6] = "Boat";
arrItemsGrp1[6] = 2;
arrItems1[7] = "Submarine";
arrItemsGrp1[7] = 2;

arrItems1[0] = "Planes";
arrItemsGrp1[0] = 3;
arrItems1[1] = "Ultralight";
arrItemsGrp1[1] = 3;
arrItems1[2] = "Glider";
arrItemsGrp1[2] = 3;

var arrItems2 = new Array();
var arrItemsGrp2 = new Array();

arrItems2[21] = "747";
arrItemsGrp2[21] = 0
arrItems2[22] = "Cessna";
arrItemsGrp2[22] = 0

arrItems2[31] = "Kolb Flyer";
arrItemsGrp2[31] = 1
arrItems2[34] = "Kitfox";
arrItemsGrp2[34] = 1

arrItems2[35] = "Schwietzer Glider";
arrItemsGrp2[35] = 2

arrItems2[99] = "Chevy Malibu";
arrItemsGrp2[99] = 5
arrItems2[100] = "Lincoln LS";
arrItemsGrp2[100] = 5
arrItems2[57] = "BMW Z3";
arrItemsGrp2[57] = 5

arrItems2[101] = "F-150";
arrItemsGrp2[101] = 3
arrItems2[102] = "Tahoe";
arrItemsGrp2[102] = 3

arrItems2[103] = "Freight Train";
arrItemsGrp2[103] = 4
arrItems2[104] = "Passenger Train";
arrItemsGrp2[104] = 4

arrItems2[105] = "Oil Tanker";
arrItemsGrp2[105] = 6
arrItems2[106] = "Fishing Boat";
arrItemsGrp2[106] = 6

arrItems2[200] = "Los Angelas Class";
arrItemsGrp2[200] = 7
arrItems2[201] = "Kilo Class";
arrItemsGrp2[201] = 7
arrItems2[203] = "Seawolf Class";
arrItemsGrp2[203] = 7



function selectChange(control, controlToPopulate, ItemArray, GroupArray) {
  var myEle ;
  var x ;
  // Empty the second drop down box of any choices
  for (var q=controlToPopulate.options.length;q>=0;q--) controlToPopulate.options[q]=null;
  if (control.name == "firstChoice") {
    // Empty the third drop down box of any choices
    for (var q=form.thirdChoice.options.length;q>=0;q--) form.thirdChoice.options[q] = null;
 }
  // ADD Default Choice - in case there are no values
  myEle = document.createElement("option") ;
  myEle.value = 0 ;
  myEle.text = "[SELECT]" ;
  // controlToPopulate.add(myEle) ;
  controlToPopulate.appendChild(myEle)
  // Now loop through the array of individual items
  // Any containing the same child id are added to
  // the second dropdown box
  for ( x = 0 ; x < ItemArray.length  ; x++ ) {
    if ( GroupArray[x] == control.value ) {
      myEle = document.createElement("option") ;
      //myEle.value = x ;
      myEle.setAttribute('value',x);
      // myEle.text = ItemArray[x] ;
      var txt = document.createTextNode(ItemArray[x]);
      myEle.appendChild(txt)
      // controlToPopulate.add(myEle) ;
      controlToPopulate.appendChild(myEle)
    }
  }
}

function selectChange(control, controlToPopulate, ItemArray, GroupArray) {
  var myEle ;
  var x ;
  // Empty the second drop down box of any choices
  for (var q=controlToPopulate.options.length;q>=0;q--) controlToPopulate.options[q]=null;
  if (control.name == "firstChoice") {
    // Empty the third drop down box of any choices
    for (var q=form.thirdChoice.options.length;q>=0;q--) form.thirdChoice.options[q] = null;
  }
  // ADD Default Choice - in case there are no values
  myEle=document.createElement("option");
  theText=document.createTextNode("[SELECT]");
  myEle.appendChild(theText);
  myEle.setAttribute("value","0");
  controlToPopulate.appendChild(myEle);
  // Now loop through the array of individual items
  // Any containing the same child id are added to
  // the second dropdown box
  for ( x = 0 ; x < ItemArray.length  ; x++ ) {
    if ( GroupArray[x] == control.value ) {
      myEle = document.createElement("option") ;
      //myEle.value = x ;
      myEle.setAttribute("value",x);
      // myEle.text = ItemArray[x] ;
      var txt = document.createTextNode(ItemArray[x]);
      myEle.appendChild(txt)
      // controlToPopulate.add(myEle) ;
      controlToPopulate.appendChild(myEle)
    }
  }
}
// -->
</script>

</HEAD>

<BODY BGCOLOR=#ffffff vlink=#0000ff>

<!-- Demonstration -->
<form name=form>
<table align="center">
  <tr>
    <td>
      <select id="firstChoice" name="firstChoice" onchange="selectChange(this, form.secondChoice, arrItems1, arrItemsGrp1);">
        <option value="0" selected>[SELECT]</option>
        <option value="1">Land</option>
        <option value="2">Sea</option>
        <option value="3">Air</option>
      </select>
    </td><td>
      <select id="secondChoice" name="secondChoice" onchange="selectChange(this, form.thirdChoice, arrItems2, arrItemsGrp2);">
      </select>
      <select id="thirdChoice" name="thirdChoice">
      </select>
    </td>
  </tr>
</table>
</form>

</body>

</html>