Object doesn't support property or method error

[Hebert]

I would like to say thanks to jkd for the quad combo script that I picked up from him about 5 or 6 months ago, (I appreciate it). I do have one error that does not affect functionality of the site, but may cause trouble later on down the line.

Error: line #637: object doesn't support this property or method
Character 1

Code:

function change3() {
for (z=0;z<=max;z++) {
document.quad.four.options[z]=null;
Line #637 if (fourth[document.quad.one.options[document.quad.one.selectedIndex].value][document.quad.two.options[document.quad.two.selectedIndex].value][document.quad.three.options[document.quad.three.selectedIndex].value][z] != null)
document.quad.four.options[z]=fourth[document.quad.one.options[document.quad.one.selectedIndex].value][document.quad.two.options[document.quad.two.selectedIndex].value][document.quad.three.options[document.quad.three.selectedIndex].value][z];
else document.quad.four.options[z]=null;
}
document.quad.four.selectedIndex=0;
}

I am unsure of what properties and methods are applying to what object. I have tried to walk through step by step and I cannot find any inconsistencies with the other references listed above this one. Any help would be greatly appreciated.

Thanks for your time.