Dirt Rally is Dirt 4.
Consider that Codemasters went through internal nightmares and massive loss of profits, forced to put out poor quality stuff with rehashed content a lot of the developers spoke out about it, the top guys running the company are no longer the same... A lot of **** went down.
Then the guys responsable for Dirt (who are genuine rally fanatics) thought this was an opportunity to make a real Rallying game that people would play for years and respect, one that would have a lasting legacy like Richard Burns Rally did with the fans, rather than being a throwaway title that people would play and then trade in (which is how they themselves described some of the previous games).
If you read any of the roadmaps regarding Dirt Rally, and also followed the company issues that came before it you will realise that Dirt Rally is Dirt 4, it's the Dirt game they always wanted to make... It's also fantastic, if a little barebones at the moment, it's still mid development.