Use the css class col-xs-6 in each column. If you want 1 columns on smaller devices and 2 columns above 767px wide, use col-xs-12 col-sm-6.
Something like this:
<div class="row"><div class="col-md-4 col-xs-6"><img alt="Responsive across devices" src="assets/img/devices.png" class="img-responsive"></div><div class="col-md-4 col-xs-6"><img alt="Responsive across devices" src="assets/img/devices.png" class="img-responsive"></div><div class="col-md-4 col-xs-6"><img alt="Responsive across devices" src="assets/img/devices.png" class="img-responsive"></div><div class="col-md-4 col-xs-6"><img alt="Responsive across devices" src="assets/img/devices.png" class="img-responsive"></div></div>