[jjones752]

The z-link acts like a parallelogram and moves the birdcages more or less straight up and down which is great for preventing unwanted roll steer but terrible for keeping constant chain tension. Granted, the radius rods and torsion arms aren't equal length so there are some intended geometric changes but for simplicity's sake let's leave that there.
A 4-link keeps the birdcages relatively in the same "clocking" attitude but the arcs of the trailing-arms/radius rods are closer to the arc of the chain so (in theory) there should be less of a variation in tension. the birdcage clocking should remain static since the arms are equal length.
A third alternative is the wishbone, which, if your front heim joint is as close as possible to the centerline of your countershaft will produce nearly zero chain tension change, and the birdcages remain fixed in relation to the arc of the wishbone instead of staying relatively vertical. There are pros and cons for all 3 setups that involve drive off the corners, bind, and a lot of stuff I don't understand but those are the basics.
My old Zonker had a wishbone setup and I never had an issue with the chain. I did break a couple of front heims and in one of those incidents dug the left wishbone into the track with unpleasant results, but I put a pair of drag link tethers on the new set and never broke another heim joint. Funny how that works...