1 (140px width each)
2
3

The problem here is that the calc should be ((100% - (140px * 3)) / 4)

In actual fact there is an 8px value, presumably for the non-existing vertical scrollbar(?), present, as:

((100% - (140px * 3) + 8px) / 4)

  <style>
  .calc { margin-top:30px; padding:20px 0; border:1px solid; }
  .calc div {display:inline-block;}
  .calc div:first-child { margin-left:-moz-calc((100% - 428px)/4); margin-right:-moz-calc((100% - 428px)/4); }
  .calc div:nth-child(2){  }
  .calc div:nth-child(3){ margin-left:-moz-calc((100% - 428px)/4); margin-right:-moz-calc((100% - 428px)/4); }
  </style>