Please make sure you have the latest version of gulp CLI. You will need to remove your current gulp global package before installing v4 in order to do an upgrade.

# Uninstall previous Gulp installation and related packages, if any
$ npm rm gulp -g
$ npm rm gulp-cli -g
$ cd [your-project-dir/]
$ npm rm gulp --save-dev
$ npm rm gulp --save
$ npm rm gulp --save-optional
$ npm cache clean

# Install the latest Gulp CLI tools globally
$ npm install gulpjs/gulp-cli -g

# Install Gulp 4 into your project from 4.0 GitHub branch as dev dependency
$ npm install gulpjs/gulp#4.0 --save-dev

# Check the versions installed. Make sure your versions are not lower than shown.
$ gulp -v
---
[10:48:35] CLI version 1.2.2
[10:48:35] Local version 4.0.0-alpha.2