For Bootstrap 4 stable:
Since beta Bootstrap 4 doesn’t depend on Tether but Popper.js. All scripts (must be in this order):
<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js"></script>
See the current documentation for the newest script versions.
Only Bootstrap 4 alpha:
Bootstrap 4 alpha needs Tether, so you need to include tether.min.js before you include bootstrap.min.js, eg.
<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"></script>