Webpack only using 1 thread even though parallel set to 4

I am trying to figure out why webpack is only using 1 thread (and 2048MB of ram) when I have set parallel: 4 in my vue.config.js file.

How can I get webpack to use more threads? This is with vue-cli v4.5.12 and webpack v4.46.0.
My system has 8 cores and 16 logical processors and 16 GB of ram.

Thanks