Expert Difficulty Setting
Complete the game using the 'Hard' difficulty setting to unlock the 'Expert' difficulty setting.

Ace Difficulty Setting
Complete the game using the 'Expert' difficulty setting to unlock the 'Ace' difficulty setting.

Unlock Extra Options
After you have completed all 18 missions, save the game. Then, load the saved game to unlock the following lots of extra options at the main menu.

Go to: Ace Combat 4: Distant Thunder Info Page | nGaming Home Page.