I can't get CSS flex to stack to up on mobile

Question

I went here and followed the steps

Border on equal height columns with Bootstrap flex

and created my own fiddle here.

I want to have a set of columns that stack 2up on mobile devices but ALWAYS keep the same height as the tallest element.

I tried to recreate the fiddle but I'm missing some basic understanding of CSS flex.

https://jsfiddle.net/galnova/z9wjnve3/

My HTML

<row class="col-xs-12 outer_wrap nill">
  <div class="col-xs-6 col-sm-3 pannel">
    <div class="col-xs-12 pannel_inner">
      <h4>Title One</h4>
      <p>
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Cras placerat, dolor eu sagittis luctus, nibh orci tristique nibh, quis facilisis nisl dolor sed purus.<br/></br/>Suspendisse mollis pretium quam, et tempor sapien congue id. Nunc suscipit eros laoreet nisi placerat dapibus. Quisque tempus magna metus
      </p>
      <button class="btn btn-secondary col-xs-12">Button</button>
    </div>
  </div>
      <div class="col-xs-6 col-sm-3 pannel">
    <div class="col-xs-12 pannel_inner">
      <h4>Title One</h4>
      <p>
Lorem ipsum dolor sit amet, consectetur adipiscing elit. Cras placerat, dolor eu sagittis luctus
      </p>
      <button class="btn btn-secondary col-xs-12">Button</button>
    </div>
  </div>
    <div class="col-xs-6 col-sm-3 pannel">
    <div class="col-xs-12 pannel_inner">
      <h4>Title One</h4>
      <p>
Suspendisse mollis pretium quam, et tempor sapien congue id. Nunc suscipit eros laoreet nisi placerat dapibus. Quisque tempus magna metus
      </p>
      <button class="btn btn-secondary col-xs-12">Button</button>
    </div>
  </div>
    <div class="col-xs-6 col-sm-3 pannel">
    <div class="col-xs-12 pannel_inner">
      <h4>Title One</h4>
      <p>
Suspendisse mollis pretium quam, et tempor sapien congue id. Nunc suscipit eros laoreet nisi placerat dapibus.
      </p>
      <button class="btn btn-secondary col-xs-12">Button</button>
    </div>
  </div>
</row>

MY CSS

.outer_wrap { /* display: flex; flex-wrap: wrap; overflow: hidden; position: relative; border: solid 1px grey; margin: 15px; */ }



.pannel { padding: 15px; display: flex; flex-wrap: wrap; }
.pannel:after,
.pannel:before {
  display: flex;
}
.pannel h4 { text-align: center; padding-top: 15px; padding-bottom: 15px; background: #eee; margin-left: -15px; margin-right: -15px; }

.pannel_inner { background: pink; border: solid 1px green; padding-bottom: 15px; display: flex; flex-direction: column; }
/* .pannel_inner p { background: red; display: flex; flex: 1; } */

.nill {
  padding: 0;
  margin: 0;
}