Thanks to Ted Culotta and FoFC 6 it looks like the Rr-20 class of dry ice reefers actually has seven doors, four on the R side and three on the L. I am not sure if there was a longitudinal partition in the center of the car or if there were small three transverse compartments accessible from only one side of the car. On page 121 of the Santa Fe Rolling Stock Reference series Vol 2, Refrigerator Cars is a photo of the four door side of SFRD 6011, which at the time of publication was the only known photo of a "four door" car.
With this new publication of both sides of SFRD 6004, it is evidence of at least two of that class with four doors on at least one side. While unusual it wasn't the first instance of asymmetrical construction. It gives credence to the possibility that there were seven doors on the bulk of that series of dry ice reefers 6004-11.
