Is your feature request related to a problem? Please describe.

Slim Framework announced 4.0.0-alpha release in previous month.
https://www.slimframework.com/2019/04/25/slim-4.0.0-alpha-release.html
Current Slim server generator based on 3 version.

Describe the solution you'd like

I think it's perfect time to migrate to Slim 4 while openapi-generator is in prestable status.