Your correct, it never did reach production, but according to Achtung Panzer one was built.. I haven't seen any pics either. It was also intended to re-arm the Wirbelwind with 4 30mm guns, again one was produced. This is the Achtung Panzer page of the Wirbelwind and Ostwind.
http://www.achtungpanzer.com/flak4.htm
As for the Modelwagen with the quad 20mm, this was the original idea for the Modelwagen, armed with the same gun as the Wirbelwind. Only one was built and then they decided to go with the 3.7cm as it fired heavier shells. Tamiya did do a kit of it years back. I did build one about 8 years ago but unfortunatly it was one of many that didn't survive. And this is the link to the Modelwagen page which includes a pic of the Quad 20mm version.
http://www.achtungpanzer.com/flakpanzer-iv-mobelwagen-sdkfz1613.htm
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