問題：
解決辦法 (http://github.hubspot.com/tether/)
Bootstrap 4 needs Tether, so you need to include tether.min.js before you include bootstrap.min.js, eg. 必須引入Tether.js,打開 https://npmcdn.com/bootstrap@4.0.0-alpha.5/dist/js/bootstrap.min.js 得到文件內容，復制新建一個文件保存再引入改JavaScript文件！
<script src="https://npmcdn.com/tether@1.2.4/dist/js/tether.min.js"></script>
<script src="https://npmcdn.com/bootstrap@4.0.0-alpha.5/dist/js/bootstrap.min.js">