The motor makes no difference as far as where the CG should be. I don't think you fully understand what CG means. CG (center of gravity) is not some imaginary line on the wing of an aircraft. The CG of an aircraft is where it balances on the chord of the wing. To move the CG Fore ("up"), you add weight to the nose. To move the CG aft ("back"), you add weight to the rear of the aircraft. The instructions are written with a range within which the plane will fly like it is supposed to, as long as the plane balances within that range. So, in other words, if the instructions say that the CG should be set at 3 1/2 to 4 inches back on the wing at the fuse, that means your plane should balance somewhere within that range. If you plane balances at 3 1/2 inches, then that is the CG of your plane. If it balances at 4 inches, then that is the CG of your plane. If your plane balances at 5 inches, then that is the CG of your plane, and you should move the CG forward to within the acceptable range by adding weight to the nose of the plane.
The best way to move the CG fore or aft is to move the weight that you already have around--i.e. the battery, receiver, and servos (if possible). Also, adding a larger, heavier engine is acceptable if the plane is too tail heavy. However, the CG range recommended within the instructions has nothing to do with what engine you are going to put on it.