Bootstrap column different sizes?

Hi everyone I've been having this problem with some of my code for this website I'm working on and I can not get the columns to be the same size? How Would I go about doing this?

<div class="col-xs-18 col-sm-6 col-md-3">
      <div class="thumbnail">
        <img src="img/plant_machinery/Uni Beams-4 (thumbnail).jpg" alt="">
          <div class="caption">
            <h4>Universal Lifting Beam</h4>
            <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Facere, soluta, eligendi doloribus sunt minus amet sit debitis repellat. Consectetur, culpa itaque odio similique suscipit</p>
            <p><a href="img/plant_machinery/Uni Beams-4.jpg" class="btn btn-info btn-xs" role="button">Preview</a></p>
        </div>
      </div>
    </div>
    <div class="col-xs-18 col-sm-6 col-md-3">
      <div class="thumbnail">
        <img src="img/plant_machinery/Sleeper Spreader-1 (thumbnail).jpg" alt="">
          <div class="caption">
            <h4>Sleeper Spreader Beam</h4>
            <p>Product code: <b>tMe-PW-Dc</b></p>
            <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Facere, soluta, eligendi doloribus sunt minus amet sit debitis repellat. Consectetur, culpa itaque odio similique suscipit</p>
            <p><a href="img/plant_machinery/Sleeper Spreader-1.jpg" class="btn btn-info btn-xs" role="button">Preview</a></p>
        </div>``

Thanks for any replies.