This should work for alpha 6. The key is the class "mr-auto" on the left nav, which will push the right nav to the right. You also need to add navbar-toggleable-md or it will stack in a column instead of a row. Note I didn't add the remaining toggle items (e.g. toggle button), I added just enough to get it to formatted as requested. Here are more complete examples https://v4-alpha.getbootstrap.com/examples/navbars/.
<!DOCTYPE html>
<html lang="en">
<head>
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css" rel="stylesheet" />
</head>
<body>
<nav class="navbar navbar-toggleable-md navbar-light bg-faded">
<div class="container">
<a class="navbar-brand" href="#">Navbar</a>
<ul class="nav navbar-nav mr-auto">
<li class="nav-item active">
<a class="nav-link" href="#">Home</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>
</ul>
<ul class="nav navbar-nav">
<li class="nav-item">
<a class="nav-link" href="#">Link on the Right</a>
</li>
</ul>
</div>
</nav>
</body>