问题：
解决办法 (http://github.hubspot.com/tether/)
Bootstrap 4 needs Tether, so you need to include tether.min.js before you include bootstrap.min.js, eg. 必须引入Tether.js,打开 https://npmcdn.com/bootstrap@4.0.0-alpha.5/dist/js/bootstrap.min.js 得到文件内容，复制新建一个文件保存再引入改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">