O.K. Buster 4ce

O.K. Buster 4ce (OKバスターフォース) is the fourth mainline O.K. Buster game and the last to be released by Sony Computer Entertainment.The title is a play on the words force and ace.

Gameplay
O.K. Buster 4ce marks the first game where Warp Gates are introduced. Warp Gates are exclusive to single-player campaign mode, and can be used to skip some levels for the player depending on the color: green Warp Gates skip over one level, blue Warp Gates skip two, and red Warp Gates skip five.

In addition to the traditional Campaign, Busting, and 2-player versus modes, the game features 4-player battles and an Eternal Mystery Buster mode. The story mode, detailed in the following section, features 16 levels for Erla to explore, with 16 bosses to fight. Busting Mode now allows players to select from four differently-sized fields. Eternal Mystery Buster is a "time attack" mode where players attempt to complete several objectives as quickly as possible. The game also, for the first time in the series, allows players to freely fight the playable roster single-player, as opposed to only being restricted in Story mode.

Story
With the advent of the Oracle Festival, Erla and Tabby have come to visit and enjoy festivities. Erla makes her way to one of the tents to watch a show, led by Lucifer as the ringmaster. As part of one of the acts, Lucifer requests a volunteer to step into the stage. With Tabby as said volunteer, he performed a disappearing act, making him vanish before the crowd. After Erla leaves the tent after the show has concluded, she realizes Tabby is nowhere to be found. Thus, Erla starts exploring the circus grounds in search of her partner.

O.K. Buster 4ce features a Starter and Regular course. The Starter course features the first three stages and does not contain any cutscenes. Erla cannot use her special attacks in story mode, but can call upon the powers of Serpent Cherub, Sayuri, Shrine Maiden, and Misa as the player advances. Unlike previous games' story modes, the gameplay rules drastically change between stages.

Due to the length of the Campaign Mode compared to other games, the game saves after every stage, allowing the player to quit anytime and then start back on their current stage without losing progress.

Festival Stage - Basic Rules Volcano Stage - Steel bricks will appear. Sea Stage - The player will encounter peculiar bricks. Sky Stage - A multiplier is added to your score, depending on how many enemies are defeated. Ruin Stage - Steel, Iron, and "peculiar" bricks will be found across the field. Outer Festival Stage - Uses the rules from the Volcano and Sky Stages. Otherworld #1 - Field wraps around horizontally. Otherworld #2 - Uses all of the aformentioned rules.
 * Stage 1: Skull-J - Played after 3 phases.
 * Stage 2: Bowlet - Played after 3 phases.
 * Stage 3: Hikiko - Played after 4 phases.
 * Stage 4: Juster - Played after 3 phases.
 * Stage 5: Serpent Cherub - Played after 4 phases. Serpent joins your party afterwards.
 * Stage 6: Popota - Played after 3 phases. Iron bricks take the place of steel bricks.
 * Stage 7: Sayuri - Played after 4 phases. Sayuri joins your party afterwards.
 * Stage 8: Griffhawk - Played after 4 phases.
 * Stage 9: Siren - Played after 3 phases.
 * Stage 10: Shrine Maiden - Played after 4 phases. Shrine Maiden joins your party afterwards.
 * Stage 11: Manticore - Played after 3 phases.
 * Stage 12: Misa - Played after 4 phases. Black bricks will appear in the field. Misa joins your party afterwards.
 * Stage 13: Schwaz - Played after 4 phases. Black bricks will appear in the field.
 * Stage 14: Diann - Played after 4 phases.
 * Stage 15: Lucifer - Played after 5 phases. You cannot perform special attacks.
 * Stage 16: Other Erla - Played after 4 phases. You cannot perform special attacks. "Peculiar" Bricks appear throughout the field. Character powers are weak. Use the Pegas O.K. to beat her, but it is possible to do so without one.