The version 4.0 is known to be universally available for all Mac OS X versions, but there is nothing mentioned about 4.2. It's specified on the official website that's released and available for downloading, but based on the error you're receiving, it appears the developer has removed compatibility with older versions of Mac OS X. There is nothing to be done in such cases except contacting the developers and explaining the problem.
You can either post in the group or send email to developers:
