>>9
Sometime in 2004, I believe. The domain name was registered on November 1, 2004, and I can't find any threads that still exist from prior to November.

However, the title of this thread (the oldest one I could find) implies that 4-ch already existed before it was started: https://archives.4-ch.net/anime/kareha.pl/1099441688/

Unless the previous Genshiken threads were on world2ch or something anyway.