Laravel 5.4 use own webpack.config.js

My problem
The docs for laravel mix says that if you want to use your own webpack.config.js you can copy it to your root folder and then edit it.
My problem is that, looking in the original webpack.config.js file, I only see five lines of code which mostly require other files. It looks a bit like a rabbit hole of requires but I couldn't find a file which looks like a real webpack.config file.
How am I supposed to use my own weback.config file without overwriting parts of the config which are needed by laravel?

What I want to achieve
I want to upgrade webpack 3 to webpack 4 without upgrading the whole laravel installation. I started by installing webpack 4. Afterwards I got a warning that the extract-text-webpack-plugin needs webpack 3.
I noticed that there is a new extract-text plugin (mini-css-extract-plugin).
Because I suppose that the old plugin is loaded somewhere in the original config I wanted to delete this part to prevent problems.

via Chebli Mohamed