If you pre-ordered the digital version of Mad Max but did not receive the Road Warrior DLC, you should be able to download this for free via the PSN store. To do this:
- Navigate to the Game Page on your PS4 dashboard
- Scroll down to the Related Items section at the bottom
- Select the Road Warrior DLC to download it
You can also purchase the DLC for free and download it via the Playstation Store menu in-game.