To unlock the four bonus cars in the game you'll need 
to do the following: 

Subaru Impreza 2000 
- Win the 2.0L Championship in Career Mode. 

Mitsubishi Lancer Evolution VI 
- Win the 1.6L Championship in Career Mode. 

SEAT Cordoba Repsol 
- Complete Challenge Mode. 

Toyota Corolla V-Rally 
- Set record time for all circuits.