Gulp 3.9 to 4 Migration

I know this has been asked many times before, but none of the answers helped me to solve my problem migrating gulp 3 to 4. We didn't necessarily have to upgrade to version 4 of gulp, but updating Node.js from 10 to 12 forced us to do so, since Node.js 12 doesn't support gulp 3 anymore. Here are just 2 of files in our build process, I think that it should be enough to understand what the problem is from these files alone, but I can add the other files if need be. And I have also removed the contents of most functions for brevity.

// gulpfile.js

'use strict';

var gulp = require('gulp');
var wrench = require('wrench');

/**
 *  This will load all js or coffee files in the gulp directory
 *  in order to load all gulp tasks
 */
wrench.readdirSyncRecursive('./gulp').filter(function (file) {
    return (/\.(js|coffee)$/i).test(file);
}).map(function (file) {
    require('./gulp/' + file);
});


/**
 *  Default task clean temporaries directories and launch the
 *  main optimization build task
 */
//gulp.task('default', ['clean'], function () {      <-- Original line, worked in gulp 3.9
function main(done)
{
    gulp.start(build);
    done();
}
exports.default = gulp.series(clean, main);

And another file:

// build-dev.js

'use strict';

var path = require('path');
var gulp = require('gulp');
var conf = require('./conf');

var $ = require('gulp-load-plugins')({
    pattern: ['gulp-*', 'main-bower-files', 'uglify-save-license', 'del']
});

//gulp.task('html-dev', ['inject'], function () <-- Original line, worked in gulp 3.9
function htmlDev()
{
    // Removed for brevity...
}
exports.htmlDev = gulp.series(exports.inject, htmlDev);     <-- this is the line that fails

//gulp.task('fonts-dev', function () <-- Original line, worked in gulp 3.9
function fontsDev()
{
    // Removed for brevity...
}
exports.fontsDev = fontsDev;

//gulp.task('other-dev', function () <-- Original line, worked in gulp 3.9
function otherDev()
{
    // Removed for brevity...
}
exports.otherDev = otherDev;

//gulp.task('clean', function () <-- Original line, worked in gulp 3.9
function clean()
{
    // Removed for brevity...
}
exports.clean = clean;

//gulp.task('build:dev', ['html-dev', 'fonts-dev', 'other-dev']); <-- Original line, worked in gulp 3.9
exports.buildDev = gulp.series(exports.htmlDev, fontsDev, otherDev);

And when I run gulp I get the following error:

    AssertionError [ERR_ASSERTION]: Task never defined: undefined
    at getFunction (F:\Dev\DigitalRural\Main\Mchp.DigitalRural.Portal\node_modules\undertaker\lib\helpers\normalizeArgs.js:15:5)
    at map (F:\Dev\DigitalRural\Main\Mchp.DigitalRural.Portal\node_modules\arr-map\index.js:20:14)
    at normalizeArgs (F:\Dev\DigitalRural\Main\Mchp.DigitalRural.Portal\node_modules\undertaker\lib\helpers\normalizeArgs.js:22:10)
    at Gulp.series (F:\Dev\DigitalRural\Main\Mchp.DigitalRural.Portal\node_modules\undertaker\lib\series.js:13:14)
    at Object.<anonymous> (F:\Dev\DigitalRural\Main\Mchp.DigitalRural.Portal\gulp\build-dev.js:61:24)
    at Module._compile (internal/modules/cjs/loader.js:936:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:947:10)
    at Module.load (internal/modules/cjs/loader.js:790:32)
    at Function.Module._load (internal/modules/cjs/loader.js:703:12)
    at Module.require (internal/modules/cjs/loader.js:830:19) {
  generatedMessage: false,
  code: 'ERR_ASSERTION',
  actual: undefined,
  expected: true,
  operator: '=='
}  

The error is in the second file, build-dev.js, and I indicate it in the code I provided. I have been trying to follow tutorials and SO questions, but to no avail. What gives?

OK, I apparently got it all wrong (yup, makes sense, from a guy that doesn't know either gulp 3 nor gulp 4 :)).
Since gulp 4 has some quite substantial changes, I had to actually rewrite the whole process (not the tasks themselves, they are more or less fine, except some here and there).
So basically, I changed the tasks to functions, used exports for some tasks to make them well known, and used series/parallel for the tasks' flow.
But I have another problem, related to the destination path, but that's a topic for another post.
Thanks everyone.