The tuned pipes increase the power on a 2 stroke engine by supercharging it when it pushes the unburned fuel mixture back into the cylinder, so increasing the pressure inside the chamber of new fuel mixture and "unburned" fuel mixture from the exhaust.
Because the four stroke has valves, the engine can not use this method as the exhaust valve will close before that can happens and the intake valve will blow out the fresh mixture during the valve overlap.
As previously noted, on the four strokes they are only silencers, but they evolved enough to have no power reduction in the engine performance and still being able to keep it quiet, but this statement is applicable to Hatori and Asano pipes, I do not have experience with any other, but I do now the standard mufflers like the OS will reduce the engine power.

Regards