In the 3D world we live in there are 6 degrees of freedom (DOF). 3 are translational and 3 are rotational. A rear axle should be able to move within 2 of those DOF's (up and down and articulating side to side). In order to have it only move in 2 ways it must have 4 degrees of restraint (DOR) (6-4=2). In real terms a link has 1 DOR, an A-arm has 2, and a MacPherson Strut also has 2. So we need either 4 links (a Panhard bar is considered a link in this discussion), 1 A-arm and 2 links, or 2 A-arms (save the struts for imports). When you look at Radius arm suspensions you can see that a radius arm setup can actually be thought of as a 5-link (2 A-arms and a Panhard). That is why it binds! If you get rid of the Panhard you can see that it still has 2 DOF's...they just aren't the ones you want. It could move up and down and side to side but not roll. You can configure links just about any way and still have 2 DOF's...they just might not be the ones you are looking for. Also remember that it takes 3 points to define a plane so if you have 4 parallel, equal length links, one of them is redundant.