In 1987 Porsche experimented with a long-wheel-base 928 called the H50 that accommodated four doors and full-size rear seats. It was built on the then recently released S4 platform with the 330 bhp V8.
Two different versions of the H50 were made, one as a station wagon and another as a hatchback. Porsche was pretty serious about testing the car and put it though 5000 miles of testing until the body was found not rigid enough to meet production standards.
Porsche hint that this car is the predecessor to the Type 970 Panamera released in April of 2009.