Gameplay
During the course of the game, the player has the opportunity to purchase 21 different planes, from models that actually exist to prototypes or relatively new aircraft that were in development (at the time of game production) and their weaponry. Every plane has two alternate paint schemes, gained by achieving a superior, or S, ranking on a stage in normal difficulty mode, by shooting down the ace pilots on each stage, or by completing the challenge modes.
The difficulty mode determines the number of enemies per stage, their AI skill and damage threshold, and how much damage the player's aircraft can sustain in total. On the hardest difficulty level, a single enemy missile can down the player's aircraft, whereas on the easiest level it would take several missile hits.
The player must purchase all aircraft and extra weapons they want to use, but only once. Identical weapons for different types of planes cannot be used interchangeably. Players gain money by destroying enemies, or selling aircraft/weapons between missions. Only the default aircraft, the F-4E Phantom, cannot be sold. It is sometimes advantageous to buy/sell different combinations of planes & weapons depending on the mission requirements outlined in the briefing, as some planes are more suitable than others for certain missions. Once all of the mission objectives are complete, the player is awarded bonus money for performing above and beyond the requirements of the mission objectives (neutralizing extra targets and assorted enemy fighters, etc.). In order to purchase all planes and weapons available in the game, the game must be completed several times.
Ace Combat 04 introduced two new additions: radio-chatter and anime-style cutscenes in which a man (who is the grown-up version of the little boy in the cutscenes) reads his memories of the Continental War over still images.
Read more about this topic: Ace Combat 04: Shattered Skies