I have been confusing myself with this topic for quite a long time. It depends on what we mean by the term "world's fastest four door sedan ".
If we just look at the top speed, then the Bentley Continental Flying Spur Speed is the fastest. On the track nothing can touch a Porsche Panamera Turbo. In a quarter mile drag race, Mercedes S65 AMG with 1000 nm of torque is unbeatable. Problem is - it is limited to 250 KM/HR. So, in the long run, the Bentley will beat it. If the speed limiter is removed, then the Mercedes would probably hit 200 MPH, so, it is quite confusing.
I have driven all these three cars but never got the chance to compare them. Amazingly if we just look at numbers then there is a fourth contender - the Cadillac CTS-V with a acceleration time of 0-100 Km/Hr in 3.9 seconds and unofficially claimed by GM as the "world's fastest four door sedan". I do doubt that because the Bentley and Mercedes would easily defeat the caddy in a drag race and the Porsche will lead on the track leaving caddy far behind.
I guess its all about the person who buys the car. If you want to win drag races then take the Mercedes Benz S65 AMG, if you just want to brag about your cars top speed then buy a Benley Continental and if you want to rule the track bye a Porsche Panamera Turbo. And lastly, if you have a limited budget - you're left with Cadillac CTS-V.
The world's fastest four door sedan should be a combination of all these and is yet to be unveiled.