[Aetae]

В современных браузерах можно так:

<form>
  <select class="test" id="test" name="mytest" multiple>
    <option>1</option>
    <option>2</option>
    <option>3</option>
    <option>4</option>
  </select >
</form>
<input type="button" value="show" onclick="alert(JSON.stringify(
 
new FormData(document.forms[0]).getAll('mytest')
 
,0,4))">