你只要用rder-md-0 order-last和 navbarcollapse就可以根据手机宽度定位了（当Navbar 是 collapsed的时候）

<div class="collapse navbar-collapse order-md-0 order-last" id="navbar4">
    <ul class="navbar-nav ml-auto">
      <li class="nav-item bg-white border-success"> <a class="nav-link" href="#">Features</a> </li>
      <li class="nav-item"> <a class="nav-link" href="#">Pricing</a> </li>
      <li class="nav-item"> <a class="nav-link" href="#">About</a> </li>
      <li class="nav-item"> <a class="nav-link" href="#">FAQ</a> </li>
    </ul> 
 </div>