Here are some more things I didn't see already mentioned:
One big reason I went with a 4-link was packaging. With Hotchkiss rear, GM put in two crossmembers in the frame--one to support front spring mounts and one for rear spring mounts, mounted at the front and back of the leaf springs.
With a 4-link, you can retain the front crossmember, but the back one you can move forward on the frame (it becomes your shock mount crossmember). This gives you room to mount a rear under the bed gas tank. :-)
Also, another reason to do this is that a 4-link doesn't wheel hop like leaf springs (that warp) under hard acceleration or braking.
Disadvantage for 4-link is you must have a positioning link to maintain side-to-side axle movement. This is a Watts link or Panhard bar that a Hotchkiss doesn't need.
