I have a G9 and find it too loud on full even in standard trim and that's me with dodgy hearing anyway. I have found with experience of Q2's, G4's and now G9 that its the positioning of the loudspeaker thats crucial. If its not loud enough, it needs to be positioning closer to the ear.
Have you done the software update to your G4? If a few years old - it apparently fixes a volume issue.
Mine is fine on iPod (turn up iPod output? But I use in-ear monitors.
Al