This is how I got Bootstrap 4.3 installed in Phoenix 1.4:

• In the project’s /assets directory:

$ npm i -D bootstrap
$ npm install --save jquery popper.js

• Add this line to your application’s app.js:

import 'bootstrap’;

• Add a scss folder under the /assets folder.
• In the scss folder, create a file called _custom.scss for bootstrap settings overrides.
• Create a file called app.scss in that folder and put these lines in it:

@import "../node_modules/bootstrap/scss/bootstrap”; (or you can import individual components)
@import "custom";

• For Bootstrap to compile, make sure you install and use the required loaders: sass-loader, postcss-loader with Autoprefixer. With minimal setup, your webpack config should include this rule or similar:

...
{
  test: /\.(scss)$/,
  use: [{
    loader: 'style-loader', // inject CSS to page
  }, {
    loader: 'css-loader', // translates CSS into CommonJS modules
  }, {
    loader: 'postcss-loader', // Run postcss actions
    options: {
      plugins: function () { // postcss plugins, can be exported to postcss.config.js
        return [
          require('autoprefixer')
        ];
      }
    }
  }, {
    loader: 'sass-loader' // compiles Sass to CSS
  }]
},
…

• Add this line to your config/dev.exs so that the contents of the assets/scss folder you created are watched while you’re developing so that changes are compiled and loaded into the assets/css folder

• Now if you fire up your phx.server and edit the sass files in the assets/scss folder, the page should refresh with the changes compiled into the relevant file(s) in the assets/css folder

In Phoenix 1.4 the bundler used for JavaScript and CSS assets is Webpack. To integrate Bootstrap into your project, you should look for instructions on installing Bootstrap through Webpack:

The Webpack configuration and sources are found in the assets folder.

Hi,
you can import it at the top of your app.css
for example in mine I have…
@import "./bootstrap.min.css";