Can anyone think why this wouldn't work?

It needs to validate a 4 digit postcode

I think im doing something wrong with ||
function numVal()
{
var pc = document.Forders.postcode.value;
if(isBlank(pc) || !isNum(pc) || pc.length != 4)
{
alert("Please enter a valid Post Code");
document.Forders.postcode.focus();
}
}