If you are looking for power for a 4V motor then check out the websites of Paul Minnaert and Zebulon or else try the Centauro Owners Group website. They have lots of tips and links to lots more tips. There is already good power in the 4V motor but lots more to be found whether you want torque, horsepower or both.
With regards to bigger pipes, by themselves they may not do much but in concert with other changes they are more suited for top end power.
Good luck whatever you do.