the satchell link is basically a triangulated 4 link with some adjustability, but with the mounting positions of the upper and lower bars reversed. this makes more room for packaging, in a car with really limited space it's a very good option. it also has the benefit of having a lower roll center tha most other options.
is it worth $4 grand? well i gues that would up the buyer and what he wants out of the car. you can similar performance fomr other systems. the hedits rear 4 link is a good option as is the TCP G-Bar rear system. a modified stock style leaf spring setup can work rather respectable as well but the ride quality won't be the same.
probably the best of all worlds would be a 3 link with a watts link or
panhard bar setup, the RRS 3 link is also a viable option now that they've redesigned the front torque arm mount too. Evolution motorsports should have their 3 link system ready to go in the next couple of months, they said they will have them for sale before Christmas this year. there is also the option of the TCP pushrod coilover system and it works well but costs more than the satchell link you posted. finally, Martz has a new 4 link with
panhard bar available as well. there are ton's of rear suspensions options for our cars now where only a few years ago there was none and i believe we have TCP to thank for that as their pushrod coilover system was the first bolt in rear
suspension option that i'm aware of.
i think the satchell link is a good system but it may be a bit over priced. my money is going to go to the TCP G-Bar triangulated 4 bar, personally, mainly because it's available for my 69 cougar, plus it's about half the cost of the satchell link.
http://totalcontrolproducts.com/gBar.html