You can use the G4 with the base off the G9 which costs about £45. The speakers do sound a lot better, but it is all relative and you'll never get hi-fi on a bike.
I do, however, prefer the G9 speakers. You also get both microphones with the G9 base as they are interchangeable, so handy if you swap lids in the future and need to use the other sort of mic.
There are some people who modify the base and install a 3.5mm socket so they can plug in their own speakers - although most people seem to do this for earphones rather than speakers so don't know what's out there.