How do I correctly import tasks from separate files into gulpfile.js/index.js?

I have been racking my brain on getting something like this to work in a simple fashion, so if someone could please help, I would be thoroughly grateful.

Using Gulp 4 and the official instructions (without any examples) for "splitting a gulpfile," I cannot find the syntax to pull in tasks from separate files. I am using gulp --tasks to test this. The instructions state:

Each task can be split into its own file, then imported into your gulpfile for composition.

Here is what I started out trying:

gulpfile.js/index.js

require('./dev');
require('./prod');

function defaultTask(cb) {
  console.log('defaultTask init');
  cb();
}

exports.default = defaultTask;

gulpfile.js/dev.js

function devTask(cb) {
  console.log('devTask init');
  cb();
}

exports.dev = devTask;

gulpfile.js/prod.js

function prodTask(cb) {
  console.log('prodTask init');
  cb();
}

exports.prod = prodTask;

What I've Tried

With these files above, when I type "gulp --list", I only see the default task, so I went searching for a way to do this without an additional plug-in. The official docs said that the tasks only needed to be imported. I tried several plug-ins to require all files within a folder or use another plug-in to pull in the files, yet those did not work with this syntax. I'm sure this is a simple problem, but its grasp is beyond me.

Thanks to anyone in advance!