Temper this with the fact that I haven't built a -4. But on the ones i've seen, it looks like the back seat pedals are just foot pads on the ends of long rods that run up to the front seat pedals. That *seems* easy to retrofit, at least the installations i've seen. Cost? Two pieces of tubing roughly about 3' long, 1/2" dia, plus a couple of 2" circles welded on the ends, and finally whatever hardware joins them to the front pedals. Does that help?
The other option is to just clamp directly to the cables (or somehow tap into the cables) where they run down the side of the cockpit. I've seen some photos of really slick installations with retractable pedals, none of which looked particularly cheap.