HTML:
<form>
<p><input id="subject" name="subject" disabled size="60"></p>
<p><input id="Second_Field" class="part">
   <input id="Third_Field" class="part">
   <input id="Fourth_Field" class="part"></p>
</form>
</ p>
JavaScript:
var updateSubject = function() {
  var outArray = [];
  for (var i=0;i<parts.length;i++) {
      if (parts[i].value !== '' ) {
        outArray.push(parts[i].value);
      }
  }
  document.getElementById('subject').value = outArray.join(' ');
};
var parts = document.getElementsByClassName('part');
for (var i=0;i<parts.length;i++) {
  parts[i].onkeydown = updateSubject;
}
</ p>