If money is no object, you can get a good handling, soft ride with a 4 link. But you will need either a panhard rod or a watts linkage to control lateral forces. To be honest that's essentially a 5 (or 6) link setup.

The stock Mustang rear suspension tries to cheat by angling the top two links in an poor effort to control lateral forces. Then they proceed to defeat the effort by using large squishy bushings and stamped U channel arms. If you run aftermarket arms that are boxed and use small, high durometer bushings you can improve it alot, but it still needs a panhard or watts linkage, imo.

To be honest, if I was to go through all the effort to custom design a 4 link I'd just put an IRS in and be done with it.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.