Some may say that using a CDN is the way to go, but for Webstorm's intellesense alone, I prefer to install Bootstrap as a dependency. There are other packages out there that you can use such as ngx-bootstrap, but this is the vanilla Bootstrap (including JavaScript) method.

UPDATE

This has been updated to use Bootstrap Beta.2 (Latest stable) as of Jan. 6, 2018.
This also works with Angular 5.
I'm aware that Beta.3 has been released, however there is a bug with ng5 and Beta.3, so sticking with Beta.2 is your best bet for now. I'll update when the bug is fixed.

1. From the root of your project, run in your terminal:

npm install bootstrap@4.0.0-beta.2 --save

2. And you need to install it's dependencies (jQuery) and (Popper.JS):

npm install jquery@1.9.1 --save

npm install popper.js@^1.12.9 --save

If you want to use SASS rather than CSS use:

npm install bootstrap@4.0.0-beta.2 --save --style=sass

Or if you want to only install for development purposes, just append --save-dev.

npm install bootstrap@4.0.0-beta.2 --save-dev, and make sure you do the same with jQuery and Popper.JS.

3. Open up your angular-cli.json file and add the style and scripts:


    "styles": [
        "../node_modules/bootstrap/dist/css/bootstrap.min.css",
        "styles.css"
      ],
      "scripts": [
        "../node_modules/jquery/jquery.min.js",
        "../node_modules/popper.js/dist/umd/popper.min.js",
        "../node_modules/bootstrap/dist/js/bootstrap.min.js"
      ],

If you're using SASS use this config:


    "styles": [
        "styles.scss"
      ],
      "scripts": [
        "../node_modules/jquery/jquery.min.js",
        "../node_modules/popper.js/dist/umd/popper.min.js",
        "../node_modules/bootstrap/dist/js/bootstrap.min.js"
      ],

4. Restart your server and you should be ready to code! (If using CSS)

With SASS you need to make one last step. Place the line below in your styles.scss file (imports bootstrap)

@import "../node_modules/bootstrap/scss/bootstrap"
Extra Sass file configuration (suggestion but not required):

Create directory in: src/styles

Create these files and place in that directory: _variables.scss and _custom.scss

Change: .angular-cli.json

      "styles": [
        "styles/styles.sass"
      ],

Import files into: styles.scss


@import "../../node_modules/bootstrap/scss/bootstrap"
@import "variables"
@import "custom"