There were prototypes, one each of the Panzer IV and the Panzer III. It was designated Panzerkampfwagen IV mit Schachtellaufwerk (also known as the BW40) and the Panzerkampfwagen III mit Schachtellaufwerk. The Pz III version used an Ausf H turret with an Ausf G hull and interleaving road wheels. The Pz IV version was an Ausf E.
Another Panzer IV testbed was the Sonderausfuhrung des Panzerkampwagen IV with hydrostatic drive driving the rear sprockets making more room the fighting compartment. The rear hull around the engine was changed. The prototype was an Ausf G but it was planned to add sloped front armour on a production version.
Is this the suspension you're talking about?
EDIT: I think you mean this: http://ww2drawings.jexiste.fr/Images/1- ... %29/p1.jpg
and this too? https://upload.wikimedia.org/wikipedia/ ... rf.svg.png