How do you get to the fourth badge in Pokemon diamond?

1. Defeat Gym Leader Fantina in Hearthome City.

2. Travel to Route 205.

3. Cross the bridge and enter Pastoria City.

4. Defeat Team Galactic in the Galactic Warehouse.

5. Go to the Pastoria City Gym.

6. Defeat Gym Leader Crasher Wake to earn the Fen Badge.