It's a Dirt game, not a Dirt Rally game... I need to adjust my expectations.
This is the most important thing.
I've only put a couple of hours into DiRT 4 so far, but I'm completely thrilled with it. It's probably the largest and most comprehensive offroad-racing package that a video game has ever offered, in my eyes.
But, Dirt: Rally it is not. If you're only interested in "pure" rally driving, and the simulation aspect is really important to you, then stick with Dirt: Rally. For me, the "simulation mode" in DiRT 4 feels close enough. I think I'm impressed more than anything... I assumed that the driving model wouldn't be super realistic, since it's a mainline DiRT game. But I think it actually feels great to play.
The graphics are fine. Not much of a step above DiRT 3; but to be fair, DiRT 3 had
really good graphics for its time (and I think not enough people appreciated that when it released in 2011). Anyway, the game's not a technical powerhouse by any means, but it runs at ~100fps in 1440p on my RX 480 without a hitch, which is good. 144hz freesync renders the game like butter.
DiRT 4 is a well-polished racing game with both quantity and quality. Not a simulator, but it hits that really nice "simcade"-type driving model that most of the other Codemasters racing games are known for (like the GRID games). It gets my personal recommendation for sure.