4 door cars generally don't have a rear wiper because as mentioned before the slope of the rear window causes to much pressure on the wiper arm. A rear window must reach a certain angle before a rear wiper is fitted. One car back here in the UK was fitted with a wiper and happened to be a 4 door sedan. It was the Ford Escort Orion Ghia SI, it turned out to be useless, the rear window demists usually keep the rear window clear.
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