V4 of BootStrap is adding Center (justify-content-center) and Right Alignment (justify-content-end) as per: https://v4-alpha.getbootstrap.com/components/navs/#horizontal-alignment
<ul class="nav justify-content-center">
<li class="nav-item">
<a class="nav-link active" href="#">Active</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Link</a>
</li>
<li class="nav-item">
<a class="nav-link" href="#">Link</a>
</li>
<li class="nav-item">
<a class="nav-link disabled" href="#">Disabled</a>
</li>
</ul>