I used NuGet to install BootStrap 4. I was also having issues with it not displaying the Dropdown on click. It kept throwing an error in jquery base on what the Chrome console was telling me.
I originally had the following
<%-- CSS --%>
<link type="text/css" rel="stylesheet" href="/Content/bootstrap.css" />
<%-- JS --%>
<script type="text/javascript" src="/Scripts/jquery-3.3.1.min.js"></script>
<script type="text/javascript" src="/Scripts/bootstrap.min.js"></script>
But I changed it to use the bundled version instead and it started to work
<%-- CSS --%>
<link type="text/css" rel="stylesheet" href="/Content/bootstrap.css" />
<%-- JS --%>
<script type="text/javascript" src="/Scripts/jquery-3.3.1.min.js"></script>
<script type="text/javascript" src="/Scripts/bootstrap.bundle.min.js"></script>