title: error: gulp.start is not a function while using gulp version 4
body: <p>Iam using gulp <strong>CLI version: 2.2.1 Local version: 4.0.2</strong>
  <strong>The node version is 12.16.3</strong>
  MY code of gulpfile.js is</p>
  
  <pre><code> 'use strict';
  var gulp = require('gulp'),
  sass = require('gulp-sass'),
  browserSync = require('browser-sync'),
  del=require('del'),
  imagemin = require('gulp-imagemin'),
  uglify = require('gulp-uglify'),
  usemin = require('gulp-usemin'),
  rev = require('gulp-rev'),
  cleanCss = require('gulp-clean-css'),
  flatmap = require('gulp-flatmap'),
  htmlmin = require('gulp-htmlmin');
  
  gulp.task('sass', function () {
      return gulp.src('./css/*.scss')
      .pipe(sass().on('error', sass.logError))
     .pipe(gulp.dest('./css'));
  });
  
  gulp.task('sass:watch', function () {
      gulp.watch('./css/*.scss', ['sass']);
  });
  
  gulp.task('browser-sync', function () {
   var files = [
    './*.html',
    './css/*.css',
    './img/*.{png,jpg,gif}',
    './js/*.js'
  ];
  
   browserSync.init(files, {
    server: {
       baseDir: "./"
    }
   });
  
  });
  
   // Default task
  gulp.task('default', gulp.series('browser-sync', function() {
    gulp.start('sass:watch');
  }));
   gulp.task('clean', function() {
     return del(['dist']);
  });
  
   gulp.task('copyfonts', function() {
     gulp.src('./node_modules/font-awesome/fonts/**/*.{ttf,woff,eof,svg}*')
    .pipe(gulp.dest('./dist/fonts'));
   });
   gulp.task('imagemin', function() {
    return gulp.src('img/*.{png,jpg,gif}')
    .pipe(imagemin({ optimizationLevel: 3, progressive: true, interlaced: true }))
    .pipe(gulp.dest('dist/img'));
   });
    gulp.task('usemin', function() {
      return gulp.src('./*.html')
     .pipe(flatmap(function(stream, file){
      return stream
       .pipe(usemin({
            css: [ rev() ],
            html: [ function() { return htmlmin({ collapseWhitespace: true })} ],
            js: [ uglify(), rev() ],
            inlinejs: [ uglify() ],
            inlinecss: [ cleanCss(), 'concat' ]
        }))
     }))
    .pipe(gulp.dest('dist/'));
   });
  
   gulp.task('build',gulp.series('clean', function() {
      gulp.start('copyfonts','imagemin','usemin');
   }));
  </code></pre>
  
  <p>The error I got is after running <strong>gulp build</strong> on the command line is:</p>
  
  <pre><code>  [15:38:33] Starting 'build'...
    [15:38:33] Starting 'clean'...
    [15:38:33] Finished 'clean' after 69 ms
    [15:38:33] Starting '&lt;anonymous&gt;'...
    [15:38:33] '&lt;anonymous&gt;' errored after 3.57 ms
     [15:38:33] TypeError: gulp.start is not a function
    at C:\Users\HARIKA\Desktop\bootstrapassign1\Bootstrap4\conFusion\gulpfile.js
  :74:10
  [15:38:33] 'build' errored after 83 m
  </code></pre>
  
  <p>I dont know how to solve the erros. I even changed some of tasks to gulp.series() as per version of gulp version 4. Can anyone help me to resolve the error? Thank you in advance.</p>