From Sajun.org

4 rigid bodies, each attached to two others with one rotational degree of freedom, to form a closed loop. Usually planar, a 4-bar is determinate if the position any two bodies are known (although there may be two solutions). One body is typically ground and does not move, so the position of only one other body is needed to find all positions.

4-bars are simple mechanisms common in mechanical engineering machine design and fall under the study of kinematics.