Decisamente meglio questo
codice:
<html>
<head>
<script>
function Riempi() {
document.longu.cerca.value="";
	for (var i=0;i<document.longu.elements.length;i++) {
		var e = document.longu.elements[i];
		if ((e.name != 'cerca') && (e.type=='text')) {
			document.longu.cerca.value+=e.value +" ";
		}
	}
//alert(document.longu.cerca.value);
}
</script>
</head>
<BODY>
<form name="longu" >
<input type="text" name="chiave1" onblur="Riempi()">

<input type="text" name="chiave2" onblur="Riempi()">

<input type="text" name="chiave3" onblur="Riempi()">

<input type="text" name="chiave4" onblur="Riempi()">

<input type="text" name="cerca">

<input type="submit" value="CERCA">
</form>
</body>
</html>