They have probably gone as far as they will go for a 4 year old product. You were lucky to get that much concession after all that time. Insulation voids are common in the industry, as the technique for injecting the liquid catalyst into a hollow cavity to make the polyurethane shell can be tricky. If any traces of oil are still on the liner or inner cabinet surface, there will be an air void in that spot.
Some of the Maytag side-by sides had this problem at first, causing the shelves in the freezer to fall out when the liner shrunk from the voids. After a series of callbacks and replacements they determined the problem was occurring during production. Usually only a few units are affected by this out of several thousand. You were the "lucky" one