If you have developed a free (no commercial benefit to yourself) application, or open source software, you may qualify for a "shared key".
The public API request can be fixed by the tuple (api key, IP address). Whenever this is done it is this tuple the one having the 4 requests/minute limitation and not the key on its own. This means that you can include a unique key in the software you have developed and each one of your users (provided they are not sharing their IP address) will experience a different 4 requests/minute limitation. Contact us in order to make your key a shared key, this is a free setting.