Enter the following icons in the password screen to be taken to the corresponding level:

Level 4: Medal, Medal, Your Plane, Enemy Plane
Level 8: Your Plane, Enemy Plane, Enemy Plane, Medal
Level 12: Bullet, Enemy Plane, Your Plane, Your Plane
Level 16: Enemy Plane, Enemy Plane, Bullet, Enemy Plane
Level 20: Your Plane, Medal, Bullet, Your Plane
Level 24: Bullet, Your Plane, Medal, Medal
Level 28: Medal, Enemy Plane, Medal, Enemy Plane