There is no DirectVR scan for Dirt4. The only head tracking that is available is vorpX emulating a gamepad stick with your headset. While rather laggy this method is passable for immersive screen mode, which is used for the game per default.
Should work without you having to do anything normally. If you have no head tracking at all, check whether gamepad controls are enabled in the game’s input options.
It should work in full VR mode too, but since there is no way to adjust the FOV beyond what’s possible in the game options that makes limited sense. The default profile settings are carefully chosen, combined with the highest possible camera FOV they work pretty well.
Default head tracking in DiRT 3 is similar, but there you have the option to use OpenTrack to emulate TrackIR input instead of vorpX for head tracking. Unfortunately D4 doesn’t have TrackIR support, so this option only works for D3.
Should be pretty straight forward to setup. Just start OpenTrack with the correct headset type selected and disable vorpX’s head tracking.