Bootstrap 4 error "Bootstrap dropdown require Popper.js", with Aurelia CLI and Require.js

Popper replaced Tether in the beta.

As such you will need to swap out tether with popper (or just add it if you never had the alpha) to the prepend section of your aurelia.json file. (Make sure you link to the UMD version seen below)

"prepend": [
   ...
          "node_modules/jquery/dist/jquery.js",
          "node_modules/popper.js/dist/umd/popper.min.js",
   ...
     ]

You will also need the bundling as expected, something like this:

      {
        "name": "bootstrap4",
        "path": "../node_modules/bootstrap/dist",
        "main": "js/bootstrap.min",
        "deps": [
          "jquery"
        ],
        "exports": "$",
        "resources": [
          "css/bootstrap.css"
        ]
      }

=Addendum=

Unlike tether, popper does not need to be prepended it seems. So you could include it like any other dependency with

 {
     "name": "popper.js",
     "path": "../node_modules/popper.js/dist/umd",
     "main": "popper.min"
 },

I uninstalled popper.js and used the version built into bs4 by using js/bootstrap.bundle.min instead of js/bootstrap.min

  "jquery",
  {
    "name": "bootstrap",
    "path": "../node_modules/bootstrap/dist",
    "main": "js/bootstrap.bundle.min",
    "deps": ["jquery"],
    "exports": "$",
    "resources": [
      "css/bootstrap.css"
    ]
  },

Add in your code.

<!-- Optional JavaScript -->
    <!-- jQuery first, then Popper.js, then Bootstrap JS -->
    <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
    <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js" integrity="sha384-ZMP7rVo3mIykV+2+9J3UJ46jBk0WLaUAdn689aCwoqbBJiSnjAK/l8WvCWPIPm49" crossorigin="anonymous"></script>
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.3/js/bootstrap.min.js" integrity="sha384-ChfqqxuZUCnJSK3+MXmPNIyE6ZbWh2IMqE241rYiqJxyMiZ6OW/JmZQ5stwEULTy" crossorigin="anonymous"></script>