Larger wheels in the G series. The wing wasn't thick enough to keep it. On some Gs you'll see the crescent-shaped bump on top of the wing. That's because when retracted the gear is angled, and only half of it is "too tall" for the wheel well. The thing with the K-4 (and G-10?) was speed and performance, so they made the MG bulges aerodynamic, and the same for the wheel well bumps. They made them longer and smoother.
There might be more behind it than that, but essentially, for speed.