Well, keep in mind I never listened to the G3 so I don't know how they've improved it. That said, I could (somewhat uncomfortably) crank the volume all the way up on my G4 with my TF10, T-Peos Altone200 and my Sony XBA-A3 IEMs. While I wouldn't want to listen to it that loud for any duration, I think it does signify that the phone may struggle with higher impedance headphones. In contrast, my One M9 and my iPhone 6 Plus I would not ever dream of reaching 100% volume. 75% or so is the max I can tolerate.
I know it isn't a test to see which phone can make a person go deaf, but I definitely noticed the volume slider reaching near the max on the G4. I think it will really come down to which IEM/headphones you choose to pair it with.
***EDIT: Where did you see that they beefed up the amp in the G4? I haven't noticed anyone reporting anything about the audio other than GSM Arena and Phone Arena. I am very curious to know!
Quality-wise the phone sounded fine to me. It sounded more along the lines of my Z3 Tablet Compact, while the iPhone 6 Plus was noticeably more flat. The G4 sounded "full" in comparison to my iPhone, which I find to be harsh at times depending on the music and which IEMs I'm using. Perhaps it was just me, but if I had to make one adjustment it would be to bring the highs just a little more forward.
Curious to know how anyone else's findings compare to mine! I'm no expert, but I've done my best for you guys
-Collin-