The official release of Bootstrap 4 finally arrived last January 2018. While we get ready to make all our content compatible with the new release, we want to make sure that you do not have to wait. Below, there is an example of the page template source that you should use to allow bootstrap in your Simplify projects.
<!doctype html>
<html lang="en">
<head>
{{header use_bootstrap="false"}}
<link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css" integrity="sha384-9gVQ4dYFwwWSjIDZnLEWnxCjeSWFphJiwGPXr1jddIhOegiu1FwO5qRGvFXOdJZ4" crossorigin="anonymous">
{{/header}}
</head>
<body>
<!-- .appc-page -->
<div class="appc-page">
{{blocks}}
</div><!-- /.appc-page -->
{{footer use_jquery="false" use_bootstrap="false"}}
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js" integrity="sha384-cs/chFZiN24E4KMATLdqdvsezGxaGsi4hLGOzlXwp5UZB1LY//20VyM2taTB4QvJ" crossorigin="anonymous"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js" integrity="sha384-uefMccjFJAIv6A+rW+L4AHf99KvxDjWSu1z9VI8SKNVmz4sk7buKt/6v9KI65qnm" crossorigin="anonymous"></script>
{{/footer}}
</body>
</html>