Having a problem with CodePen and Bootstrap 4 , won't render a simple button, yet grid works fine:
<button type="button" class="btn btn-primary">Primary</button>
This is because you're only pulling in the grid element of the Bootstrap CSS. It would seem that CodePen's existing BS4 is picking the wrong Bootstrap file.
In your pen
https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.0.0-beta.2/css/bootstrap-grid.css
Use this CDN of the full Bootstrap instead:
https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta.2/css/bootstrap.min.css