Soon after the Lindberg kit was released, there was a resin 4 door body done of the Dodge 330. It was made by a modeler out of Colorado, I can't remember his name or company name. I was lucky enough to get one. Every once in a blue moon one shows up on eBay. It should not be a hard conversion if you're so inclined.
Model Car World/MCW also does some early B body two door sedans that can be converted to four doors, too.
Tom Coolidge has the 61 Plymouth four door. Modelhaus has the 60 Plymouth four door. These were the first unibodies.