TruTrack Retrofit
I retro-fitted a TruTrack Vizon to my 20 year-old -4. Very straight forward modification. To simplify things, I did fabricate a custom set of angle brackets for the elevator bellcrank mount out of oversized stock (1 1/2" x 1 vs stock 1 x 1). This provided bigger "feet" and mitigated edge distance issues with the -4 rivets on the floor ribs. I mounted the roll servo under the forward, removable floor under the front seat. TruTrack has an excellent installation bracket for the roll servo and nice CAD drawings available. You can down-load the drawings to envision the installation. Stein did his usual first class work building the harness, and TruTrack product support is outstanding. You will have to do some control stick re-wiring, if you want full functionality (quick disconnect); unless you already have a spare wire available for this purpose.
While not strictly required, it's a lot easier to install everything if you pull all of the cockpit floors. De-riveting the aft floors and baggage compartment is quite simple and just about as fast as unscrewing them if you installed anchor nuts in lieu of avex rivets. Start in the baggage compartment and work forward, since you can unbolt the aft cockpit seatbelt mounts to gain access to all of the rivets. A 12" drill bit comes in handy for access. Do use some care drilling out rivets, as the rivets utilize the main fuselage longerons. Van's sells the rivets and any hardware you may need and you'll need some dome head and some countersunk, if airplane is built to plans. Also a good time to consider treating yourself to new harnesses, and a thorough inspection and lubrication of your flight control system, since you've got everything taken apart if you go this route. Hardware call-outs for this part of the flight control system are on Drawing 28.
I chose a stand-alone AP for redundancy, but if you already have an EFIS of some type, it's just as easy to utilize the same servo configuration and use one of those units to drive your auto-pilot.
Drop an email or PM if you'd like photos or have any questions if you decide to go the TruTrack route.
Good luck!
Vac