Please follow strictly, it may solve your problem:
1. make sure your Project Cars gets the default button layout
2. close your Project Cars on PS4
3. logout all all your users
4. delete all the configurations in Cross Hair,
and add the preset Project Cars in it, do not change any settings,
then save.
5. turn off your PS4's controller before connecting.
6. Firstly, connect the PS4's controller (USB PORT1) and
the wheel (USB PORT2 or 3) to Cross Hair, and then connect Cross Hair to console. A breathing color means it's working well.
7. During the power-on procedure, do not press the PS/GUIDE button on contorller, unless LOGO of Cross Hair is breathing.
8. Try the buttons on your G25 and the shifter in main UI of PS4
9. then try the buttons on your G25 and the shifter in the game(Project Cars)