The original prop in your plane is designed to give good pulling power... kind of like bottom end torque in a good old truck but not a lot of speed. This is helpful with little hand tossed electrics as they need the initial pull to stay airborn. The prop you bought is going to fly the plane faster but will lack some of that pulling power. If you have some experience with the plane and know what it will do when you release it you may be able to get away with flying the new prop without incedent but it will be easier to fly this plane with the 8x4 prop.