Código HTML:
<script type="text/javascript">
var fill = function (){
var obj = document.getElementsByTagName("input");
var focused = obj[obj.length-1];
focused.value = "new value!!";
}
</script>
<input type="text" value="5" />
<input type="text" value="4" />
<input type="text" value="3" />
<input type="text" value="1" />
<button onClick="fill()">add text to field</button>