ti faccio un esempio
codice:
<script language="javascript">
function Media(){
var val,ff=document.NomeForm
var n=0
var tot=0
for(var i=1;i<=4;i++){
val=ff.elements['NomeText'+i].value
if(!isNaN(val) && val!=''){
n++
tot+=parseInt(ff.elements['NomeText'+i].value,10)
}
}
if(n>0)ff.NomeTextRisultato.value=(tot/n)
else ff.NomeTextRisultato.value=''
}
</script>
<form name="NomeForm">
<input type="text" name="NomeText1" onchange="Media()">
<input type="text" name="NomeText2" onchange="Media()">
<input type="text" name="NomeText3" onchange="Media()">
<input type="text" name="NomeText4" onchange="Media()">
<hr>
<input type="text" name="NomeTextRisultato" readonly="true">
</form>