You could increase speed by installing different pulleys on the crankshaft and transmission, however this is not recommended by Honda, will void your warranty, and probably wouldn't get you all the way to 4 mph.
You could also try increasing the engine speed to 3000 RPM (if it's not already there), but don't do this unless you have a good tachometer. You could blow the engine up or cause the blades to fracture if you set the speed to high. Oh, and this is also not recommended by Honda. Making any modifications to the mower will void your warranty.
Sorry. :frown: