The motors sound sweeter with the G3 as they are a much more precise instrument.
They are made of a lot more parts than the G4 and are very finnicky. The G4 is basic to tear down and is hard to go wrong, although fiddly when refitting the cable to the cam -that's the worst part.
A guy I know who used to make go carts and motorised bikes with Vicata 160s didn't particularly care for the plastic carburetor and always used the G3 but when he fixed a neighbour's Powertorque mower be had a partial about face and thought they were a clever design for what they are.

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!