<script>
function controlla(obj) {
// elimino eventuali spazi prima e dopo
v = obj.valore.value.replace(/^\s*/,"").replace(/\s*$/,"");
v1 = obj.valore1.value.replace(/^\s*/,"").replace(/\s*$/,"");
return (v.length>=4 && v1.length>=4)
}
</script>
...
<form onsubmit="return controlla(this)">
<input type="text" name="valore">
<input type="text" name="valore1">
<input type="submit">
</form>

ciao