Originally Posted by gigaplex

Uh... what? The drivetrains are interchangeable between the 3 and 4. Other than the front spool on the TC4 vs the diff in the TC3 they're basically identical. The entire gearbox is directly interchangeable. Suspension parts are somewhat compatible too. The arms aren't because they use a different size inner hinge pin, but the hubs and knuckles are.