I believe the number "4" is bad and usually skipped in Japanese culture. Not sure of the validity of that statement, I just think I read that in a few places.
Makes sense, Lumix went from the LX3 to the LX5, and the gf3 to the gf5.
So we should probably take the 4 off the table.
Also, like others have said, a lower number indicates higher end model.