Gulp 4 Config

gulpfile.js

const gulp            = require('gulp'),
      gulpPug         = require('gulp-pug'),
      gulpSass        = require('gulp-sass'),
      gulpBrowserSync = require('browser-sync').create();
// Style syntax
const syntax = "scss";

// Gulp tasks
// Create folders structure
gulp.task('folders', function () {
  return gulp.src('*.*', {
      read: false
    })
    // src
    .pipe(gulp.dest('src/img'))
    .pipe(gulp.dest('src/vendors'))
    .pipe(gulp.dest(`src/styles/${syntax}`))
    .pipe(gulp.dest('src/js'))
});

// Compile pug files into HTML
gulp.task('pug', function () {
  return gulp.src('src/*.pug')
    .pipe(gulpPug({
      "pretty": true
    }))
    .pipe(gulp.dest('src'));
});

// Compile style files into CSS
gulp.task('styles', function () {
  return gulp.src(`src/styles/${syntax}/style.${syntax}`)
    .pipe(gulpSass({
      includePaths: [`src/styles/${syntax}`],
      errLogToConsole: true,
      outputStyle: 'compact'
    }))
    .pipe(gulp.dest('src/css'));
});

gulp.task('browser-sync', function(){
  gulpBrowserSync.init({
    server: {
      baseDir: "src/"
    },
    notify: false,
    reloadDelay: 500,
    open: false
  });
});

// js
gulp.task('scripts', function () {
  return gulp.src('src/js/*.js')
    .pipe(gulp.dest('src/js'))
});

// Serve and watch sass/pug files for changes
gulp.task('watch', function () {
  gulp.watch(`src/styles/${syntax}/*.${syntax}`, gulp.parallel('styles'));
  gulp.watch('src/*.pug', gulp.parallel('pug'));
  gulp.watch('src/index.html').on('change', gulpBrowserSync.reload);
  gulp.watch('src/css/style.css').on('change', gulpBrowserSync.reload);
  gulp.watch('src/js/*.js', gulp.parallel('scripts')).on('change', gulpBrowserSync.reload);
});

// Main GULP Task
// Create folders structure
gulp.task('folders', gulp.series('folders'));
// Task for work
gulp.task('default', gulp.parallel('browser-sync', 'watch', 'styles', 'pug', 'scripts'));
// Task for build a project
// gulp.task('build')

package.json

{
  "name": "app",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "Aleksandr Tereshchuk",
  "license": "ISC",
  "devDependencies": {
    "browser-sync": "^2.26.5",
    "gulp": "^4.0.2",
    "gulp-pug": "^4.0.1",
    "gulp-sass": "^4.0.2"
  }
}