There is no cost for version 4.0 of the software because it's not available anymore. You can't purchase it as it doesn't appear on any of the websites visited, including the official one.
If you insist on using the application, then access the official website and send an email to the developers. They should clarify the situation with the software's availability, price, alternatives, etc.
Use the following form to send them an email. Make sure to use a valid email to receive replies.
