Problem on installing bootstrap 4 in phoenix 1.4

Hello, I have read some post on internet like this:
https://medium.com/wecode-ng/how-to-install-bootstrap-on-phoenix-1-4-project-c8aa724dcdeb

but I still have problems to install bootstrap 4 in phoenix 1.4.

Step 1:

I went asset folder and run this commands

npm install --save  bootstrap
npm install --save jquery popper.js
npm install — save-dev file-loader

Step 2:

I went app.js file and added these lines:

import css from "../css/app.scss"
import "bootstrap"; 

Step 3:

Then I renamed the app.css to app.scss and added these lines:

@import "/css/fontawesome.min.css";
@import "~bootstrap/scss/bootstrap";

or

@import '../node_modules/bootstrap/scss/bootstrap';
@import '../node_modules/font-awesome/css/font-awesome.css';

Step 4

I edited my webpack.config.js

const path = require('path');
const glob = require('glob');
const MiniCssExtractPlugin = require('mini-css-extract-plugin');
const UglifyJsPlugin = require('uglifyjs-webpack-plugin');
const OptimizeCSSAssetsPlugin = require('optimize-css-assets-webpack-plugin');
const CopyWebpackPlugin = require('copy-webpack-plugin');

module.exports = (env, options) => ({
  optimization: {
    minimizer: [
      new UglifyJsPlugin({ cache: true, parallel: true, sourceMap: false }),
      new OptimizeCSSAssetsPlugin({})
    ]
  },
  entry: {
      './js/app.js': ['./js/app.js'].concat(glob.sync('./vendor/**/*.js'))
  },
  output: {
    filename: 'app.js',
    path: path.resolve(__dirname, '../priv/static/js')
  },
  module: {
    rules: [
      {
        test: /\.js$/,
        exclude: /node_modules/,
        use: {
          loader: 'babel-loader'
        }
      },
      {
        test: /\.scss$/,
        use: [
          MiniCssExtractPlugin.loader,
          {
            loader: 'css-loader',
            options: {}
          },
          {
            loader: 'sass-loader',
            options: {}
          }
        ]
      }
    ]
  },
  plugins: [
    new MiniCssExtractPlugin({ filename: '../css/app.css' }),
    new CopyWebpackPlugin([{ from: 'static/', to: '../' }])
  ]
});

after that I run the iex -S mix phx.server but it can’t find app.scss and loads bootstrap.

how can I fix and install bootstrap 4 in phoenix 1.4?

Thanks