4 workers is the sweet spot for most wsgi applications. Typically you will scale above that by running additional complete gunicorn instances (managed by some kind of external scaling/load balancing thing). If it makes sense for your application to run more than 4 workers in one gunicorn instance (ie. you have long running requests that only wait on something) then gunicorn (and in fact, "normal" wsgi itself) probably is not the right solution for your application.

Edit: somewhat typical issue with scaling python applications is that just throwing additional workers at the problem in the same instance of the wsgi container (including separate uwsgi processes) stops being effective fairly soon and makes the problem worse.