G9 can connect up to 9 people. G4 can connect up to 4 people.
There's some newer tech toys in the G9. G9 can 'bump' to pair. I do not think the g4 can do that.
Both can be programmed using your computer and Scala software.
I have the G4 and have used the G9. As far as I can tell the sound quality is the same.
If I had to re-buy I would get the G9