In 1986 Porsche and German aftermarket shop AMG built a prototype custom 928 four-door saloon. One cannot help but conjecture that this variant may have been a prototype for a new 928 that would have created a completely new market niche, further distancing it from the 911 but probably ended as the basis for the Panamera launched in 2009.
According to current Porsche designer Harm LaGaay, this rare and unusual car was delivered to Heinz Prechter, founder, and chief executive of ASC (American Sunroof Corporation), whose large automotive aftermarket firm, headquartered just south of Detroit, enjoyed a close relationship with Porsche. The workmanship of this conversion is impeccable, being equal or better of the legendary fit and finish of a new Porsche. The entire cabin, for instance, is lined in sumptuous burgundy leather to match the exterior paintwork.
This unusual factory custom Porsche was offered at no reserve at the RM Monterey auction, Aug. 16, 2002, and sold for $44,000, nearly four times the going rate for same-vintage 928s.
It wouldn't be the last time the 928 would be used for a four-door, either. In the late 1980s, Porsche itself tested a four-door. Under the direction of Porsche's design director Ulrich Bez, stylist Harm Lagaay penned the 989, which was much more of a true four-door than any of the one-offs that preceded it. While the prototype never made it to production, many of its design elements did: the canted headlamps were used on 1993 to 1998 "993" generation of the 911, and the wraparound tail lamps certainly influenced those of 1998 to 2005 "996" 911.
Sources: @motor1com @classicdriver, Collectible Wheels (2018)