Today it is pretty commonplace to see a 4 door Porsche on the road, and it is not uncommon for even ardent Porschephiles to accept, and even own one. This is a far cry from what was once considered a heretical thought. Porsche worked very hard in the early days to become a world leader in competition-derived sports cars. And it succeeded. Like Ferrari and Lotus, road cars were more of a funding source for competition, so more practical transportation was not the goal. However, once there was a modicum of success, it was inevitable that discussions about more practical variations would take place to varying degrees of seriousness. They always begin with cabriolet and fastback versions, but eventually the conversation turns to 2+2 or true 4 seaters or 4 doors. Porsche did all of the above, but like other true competition-based marques, 4 doors was not in the DNA.
There was a Type 530 prototype that Porsche produced in 1952 which had a longer wheelbase, proper rear seats, and a higher roof line. It never went further than the prototype. The most well-known early Porsche 4 door was actually a conversion of a 1967 911. It was commissioned from Troutman-Barnes for Texas Porsche dealer William Dick. Not exactly a factory effort. Discussions surely took place in the 1970s, but nothing made it to prototype stage. The first official factory effort to see the light of day was the Porsche 928 which was a birthday present for Ferry Porsche. It featured two miniature rear doors, and was more of a shooting brake in appearance. At the time, the 928 was thought to be the future, and several concept variations were developed. None made it to production, but we covered it here in our 2018 trip to the Petersen Museum where it is on exhibit. The 989 prototype of 1988 was an official effort at a large platform front-engine sedan, that was cancelled due to slumping sales of the 928.
In 2002 Porsche produced the Cayenne, which is technically the first 4 door Porsche to reach production, but it is an SUV. 20 years after the 989, in 2009, Porsche finally launched the Panamera as the first production 4 door Porsche sedan. DNA eventually evolves.