I was looking into a 4-link (4-bar, same thing) setup for a different project at one time, and a rod shop that sets up tons of them told me to go with a triangulated 4-link instead of the parallel setup. This is because the triangulated setup is much easier to get the angles right, and adjustments can get you back in line if it isn't 100% square. The parallel setup is supposed to be much more finicky.
I would check your prices though, as when I was looking, the cost of the kit plus the coilovers would be much more than the cost of new leafs. However, if you want the adjustability in both ride height and quality, the 4-link is the way to go.
Another note on axle flips, there is nothing wrong with them if they're done correctly, but they can give a very severe drop. You might find that with new springs that the ride height and quality is fine. What year is your truck? Have you checked a local junk yard for used springs?
