What I used in my -4
I have a 3.5 gallon system from Smoking Airplanes, which I mounted in my baggage compartment behind rear seat. I have 160HP, with Vetterman 4 pipe system, 2 injectors installed on 2 of the pipes. If you have a 4 pipe exhaust, you will get some "puffing" due to pulse of the firing in the individual pipes...no big deal to me. I mounted my momentary on button in the end of my throttle handle to make it really easy to use. I can pull the tank out in less than 5 minutes if I need all of the baggage compartment, but even with tank installed, I can put 2 soft bags on top of it and travel for the weekend. Just be aware with a heavy backseater and 3.5 gallons of oil, your CG could be well AFT. I usually only carry a gallon or two for the occasional "smoke pass". There are 6 of us at our home field all using the same type system, and everyone seems happy with it.
8A7 / Advance NC