Bootstrap Navbar list items/links not showing

In Bootstrap 4, the navbar-expand-* class is needed to show the horizontal navbar, otherwise it will collapse into it's mobile state.

Make sure you've referenced it properly in the Codepen as https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css

Or, use Codeply, and it will include the all the relevant libraries for you: Bootstrap 4 on Codeply

Here's the Navbar with Bootstrap 4.0.0: http://www.codeply.com/go/SmEyy7rg5S

<nav class="navbar navbar-expand-md navbar-light bg-faded">
    <button class="navbar-toggler navbar-toggler-right" type="button" data-toggle="collapse" data-target="#navbarNavAltMarkup" aria-controls="navbarNavAltMarkup" aria-expanded="false" aria-label="Toggle navigation">
        <span class="navbar-toggler-icon"></span>
    </button>
    <a class="navbar-brand" href="#">Navbar</a>
    <div class="collapse navbar-collapse" id="navbarNavAltMarkup">
        <div class="navbar-nav">
            <a class="nav-item nav-link active" href="#">Home <span class="sr-only">(current)</span></a>
            <a class="nav-item nav-link" href="#">Features</a>
            <a class="nav-item nav-link" href="#">Pricing</a>
            <a class="nav-item nav-link disabled" href="#">Disabled</a>
        </div>
    </div>
</nav>

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Related: Disable responsive navbar in bootstrap 4

your reference to their css file is not what is listed on their website. use the following:

<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">

your java script file is also not what is listed:

<!-- Latest compiled and minified JavaScript -->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>

here is where you can find the latest links to their most up-to-date CDN's: http://getbootstrap.com/getting-started/

you are have probably copied that from an experimental place. (it says alpha within the file name so there are more than likely some bugs)