HPI makes Mod 1.0 spur gears for the Nitro RS4. They don't have a 48-tooth spur gear specifically, but they have 37, 39, 41, 43, 44, 45, 46, 47, 49, and 52-tooth spur gears. You can cut the tabs off the back of the RS4's spur gear, widen the hole in the middle to fit the GT-4's spur gear carrier, and use the GT-4's spur gear carrier as a template to drill screw-holes into the RS4's spur gear.
