Evidently, the RS4 is a response to the similarly styled Yamaha YZF-R125 and upcoming KTM Duke 125. The mini Aprilia is powered by a reworked variation of the liquid-cooled, dohc, four-valve Single that was originally developed for the second-generation Scarabeo scooter. It is fed by Marelli fuel injection via a 32mm throttle body, and the three-piece, pressed crankshaft turns on plain bearings. Compressed at 12.5:1, the engine generates 15 hp at 10,500 rpm and 8.5 foot-pounds of torque at 8000 rpm. At 295 pounds dry, the RS4 125 is not as light as the 125 two-strokes of yesteryear.