There's an MIT Press book.[1] Chapter 6, "Leg Design by Four Bar Linkage Synthesis" covers the four-bar linkage. They were struggling to get enough working volume for the foot out of the four bar linkage without the mechanism becoming too bulky. They wanted to be able to move horizontally using only one actuator per leg, which allowed manual backup control.

It's an interesting piece of history, but there's no need to do it that way today.

[1] https://www.amazon.com/Machines-That-Walk-Adaptive-Suspensio...