Just got it, and first installed it on my 2010 Mac Pro | 6 Core 3.33 GHz | Radeon HD 7950 Mac Edition | 24 GB RAM | macOS 10.14.4
This thing is connected to a 1080p HD TV.
During startup the game mentions that this Mac isn't fast enough. However Dirt Rally and F1 2017 mentions the same, but this Mac handles them fine with Ultra grfx settings (1920 x 1080)
Thx to the Radeon 7950, it seems to run the game fine at Ultra Settings (a little less in the AA), though I only tried the intro-Australia stage.
However I cannot find the benchmark tool in Dirt 4 like there is in Dirt Rally and F1 2017.