Jeffrey Way just released Laravel Mix 4. This version uses Webpack 4 and claims to be faster than the 3 version.
Laravel 5.7 has been just upgraded to Mix 4 version.
Please check out the full v4 release notes before to you upgrade your existing Laravel projects to Mix v4.
Here you can found some helpfull notes about how to upgrade your existing project to v4
Upgrading to Laravel Mix 4 in Laravel Projects by Ravi Teja