ke ne dici di questo?

<html>
<head>
<script>
function controlla() {
for (var i=0;i<=4;i++) {
if (document.myform.elements[i].value == "") {
alert("manca il valore del campo n. "+(i+1));
}
}
}
</script>
</head>
<body>
<form name="myform">
1. name <input type="text" name="name">

2. mail <input type="text" name="mail">

3. y name <input type="text" name="yname">

4. y mail <input type="text" name="ymail">

<input type="button" value="go" onClick="controlla()">
</form>
</body>
</html>