вставь clearfix
<div class="container">
<div class="row">
<div class="col-md-3 col-sm-6"></div>
<div class="col-md-3 col-sm-6"></div>
<div class="clearfix visible-sm"></div>
<div class="col-md-3 col-sm-6"></div>
<div class="col-md-3 col-sm-6"></div>
</div>
</div>