Prøv om ikke den her skulle virke.
<html>
<head>
<title></title>
<script>
function tjeckDu(navn,hvorLang){
if (document.f.felt1.value.length==2){
document.f.felt2.focus();
}
}
</script>
</head>
<body onLoad=\"document.f.felt1.focus();\">
<form name=\"f\">
<input type=text name=\"felt1\" onkeyup=\"tjeckDu(this.name)\" maxlength=\"4\">
<br>
<input type=text name=\"felt2\" onkeyup=\"tjeckDu(this.name)\" maxlength=\"2\">
</form>
</body>
</html>
http://www.fotx.net/nikolajdu