For Yonex racquets, a G4 has a circumferance of 85mm. A G5 will be about 80mm in circumferance. I use both G4 and G5, as I hold my racquet loose for faster turning in my hand. A G3 or G2 would slow the turning in my hand.
This type of question must also take into account the size of the hand holding said grip. For example, a G3 feels small to me but then I wear L or XL gloves... Notice also that some grip sizes are not distributed in all countries... Cheers, Mike