There is no single official repository, as there is no way to formally judge whether any existing repository is official or not. :-)
Since YAMI4 is an open-source project, it is perfectly OK to fork it and to create as many repositories as people like. Note, however, that it is up to the maintainers to keep any given repository up to date and to keep their forks in a healthy state.
There is also a download site here:
http://inspirel.com/yami4/It looks like the version available there (1.8.0) is newer than the one on GitHub (1.6.0) or at least this is what version numbers say.
Regards,
--
Maciej Sobczak *
http://www.inspirel.com