Gradius III and IV

Gradius III and IV is a compilation game combining ports of the arcade versions of Gradius III and Gradius IV. Note that the SNES and arcade versions of these games are quite different and are therefore not the ones on this compilation.

Controls

 * psx: S: Power Up
 * psx: X: Fire
 * psx: T: Rapid Fire and Missile
 * psx: C: Select/Missile
 * psx: Start: Pause
 * psx: Neutral Lstick: Moving cursor and controlling fighter

Gameplay Modes

 * 1 Player/2 Players: The typical arcade game mode. 2 player gameplay will require two controllers. After Player 1 loses a life, Player 2's turn arrives, and the cycle goes on until both players have no lives remaining.
 * Stage Select: Allows you to start the game from a selected stage. For Gradius III: Only stages that you have reached will be available in stage select. For Gradius IV: You must complete the game to unlock this option.
 * Option Mode: Allows you to select game options. Options chosen in this mode will only affect the game you're in.
 * Main Title: Brings you back to the main menu which allows you to choose between Gradius III and Gradius IV.

Option Mode

 * Game Level: Lets you select a difficulty for the game you're in. There are 6 difficulty levels for Gradius III (Easiest - Very Hard) and 8 difficulty levels for Gradius IV (Easiest - Hardest).
 * Extend: Allows you to modify the amount of points required for a bonus.
 * Player: Determines the amount of lives you start with.
 * Power-Up: Editor's note: I'm still not sure what this is. Someone knows?
 * Adjustment: Adjust the position of the screen.
 * Button Configuration: Speaks for itself.
 * Sound Test: A jukebox for all the great tunes from Gradius.
 * Save/Load: Allows for saving and loading of high scores and progress for Stage Select.

Options Exclusive to Gradius III

 * Wait Level (Off/Level1/Level2): The PS2 processor is much faster than the original arcade hardware, therefore the amount of lag present on the original arcade game can be toggled on and off.

Options Exclusive to Gradius IV

 * Sound: Allows selection of Stereo or Mono sound.
 * Screen: Allows selection of Arcade or Full-size views.

Walkthrough
Since the games are merely arcade ports, you can refer to the original game's pages. For convenience, I'll link to them right here for you:

Gradius III

 * Stage One: Desert Stage
 * Boss 1: Goliath
 * Stage Two: Bubble Stage
 * Boss 2: Bubble Eye
 * Stage Three: Volcano Stage
 * Boss 3: Core Mark III
 * Stage Four: Turbo Stage
 * Stage Five: Moai Stage
 * Boss 4: Dogas/Vaif
 * Stage Six: Cell Invasion Stage
 * Boss 5: Gregol
 * Stage Seven: Lava Stage
 * Boss 6a: Wyvern
 * Boss 6b: Vulture Dragon
 * Stage Eight: Plant Stage
 * Boss 7: Choking Weed
 * Stage Nine: Crystal Stage
 * Boss 8: Lizard Core
 * Final Stage: Final Stage

Secrets

 * To reset the game without having to press reset, simply press psx: L1, psx: R1, psx: Start and psx: Select at the same time.