Looking at the docs at https://getbootstrap.com/docs/4.2/getting-started/introduction/#js you can see that they import the following:
<script src="https://code.jquery.com/jquery-3.2.1.slim.min.js" integrity="sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN" crossorigin="anonymous"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js" integrity="sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q" crossorigin="anonymous"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js" integrity="sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl" crossorigin="anonymous"></script>
Note the naming: jquery.slim.min.js, umd/popper.min.js!
Therefore I used the following in my .angular-cli.json:
"styles": [
"../node_modules/bootstrap/dist/css/bootstrap.css"
],
"scripts": [
"../node_modules/jquery/dist/jquery.slim.min.js",
"../node_modules/popper.js/dist/umd/popper.min.js",
"../node_modules/bootstrap/dist/js/bootstrap.min.js"
],
After that it seems to work now.