well lets look at this this way.
gear 2: pumping your blood in your veins to it's limit, making it rush very fast, using his rubber powers so his heart wont explode. getting real fast.
gear 3: using your rubber powers so your bones can enlarge with air or smth without breaking / shattering them.
so gear 4 should be something related to using your body to its limit, adjusting it and taking advantage of your rubber body. i honestly don't know what else can he do to his body.