intenta con esto
<script language="JavaScript">
var nav4 = window.Event ? true : false;
function acceptNum(evt){
var key = nav4 ? evt.which : evt.keyCode;
return (key <= 13 || (key >= 48 && key <= 57));
}
</script>
<script language="Javascript">
function promedia(form)
{
a=0;z=0
if(form.Dato1.value!=""){z++
a+=parseInt(form.Dato1.value)}
if(form.Dato2.value!=""){z++
a+=parseInt(form.Dato2.value)}
if(form.Dato3.value!=""){z++
a+=parseInt(form.Dato3.value)}
if(form.Dato4.value!=""){z++
a+=parseInt(form.Dato4.value)}
form.rs.value=a/z
}
</script>
y los datos los lee desde aca:
<form>
<input type="text" name="Dato1" onKeyPress="return acceptNum(event)"><br>
<input type="text" name="Dato2" onKeyPress="return acceptNum(event)"><br>
<input type="text" name="Dato3" onKeyPress="return acceptNum(event)"><br>
<input type="text" name="Dato4" onKeyPress="return acceptNum(event)"><br>
<input name="rs" type="text" value="Promedio">
<input onclick="promedia(this.form)" type="button" value="Promedia">
</form>
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