The Big John is the fourth boss of Metal Slug 4.
The Big John was originally an amusement park robot, but its control was taken over by the Amadeus Syndicate and converted to a weapon, turning everyone in the park into zombies or mummies using a virus similar to what the purple gas and The Ten Commandments of Moses used.
It tries to turn the player into a zombie which, if successful, could end up being its undoing, as this opens up the possibility for the player to attack via the powerful virus vial "grenade".
The Big John is composed of three phases of attacks.
- A metal claw is suspended at the top of the screen. It will move sideways until it attacks when it stops right above the player. It will attack either by descending to smashing the player or by shooting an electrocuting laser downwards but also you can get on the claw when it strikes down only for a few seconds until it retreats back up.
- After the metal claw is destroyed, the opening will periodically release purple energy spheres to attack the player. Once the metal claw is destroyed, the Big John's actual face will be revealed once its metal claw from previous phase is destroyed or after the initial face mask suffers sufficient damage. From the mouth of the actual face until the end of the battle, a Rebel soldier will periodically jump out to aid the Big John in its battle.
- The Big John's eyes will turn blue, and its second hand appears from the right side of the screen, attacking by sending a rocket to the left side of the screen and a large number of propelled virus vials that turn the players into zombies upon contact. Blue virus vials contain an item, which is either a Heavy Machine Gun, Rocket Launcher, Enemy Chaser, a mummy antidote or a first-aid kit. The rocket can be destroyed with sufficient damage. Players can jump onto and walk on the rocket without being killed.
Big John can be purchased as a unit in Metal Slug Defense for 300 medals. It is the only defensive boss unit, and costs 600 AP to deploy. It fires missiles as its primary attack, and attacks close-range units by releasing virus vials; it summons its laser cannon from the sky as its special, and uses it to cover a relatively short area in front of it.
Its missiles deal high damage have very long detection range and infinite range once fired. This in combination with its fairly high health and the fast running speed of its builder make it one of the most effective units in the game. However, it cannot hit even low-flying air units with its missiles, leaving it largely defenseless against air units, which generally sit safely outside the range of its special. If inadequately defended, melee units - particularly Green Hazmat Soldiers and Smashers, which can deploy units behind/hover over the missiles - can shut it down by forcing it to rely on its weak close-range attack. Big John's builder is also vulnerable to penetrating shots from units like Big Shiee, which can give the opponent large amounts of AP while wasting your own.
- After exactly 15 shots, the big face breaks apart, revealing the war machine.
- The sound emitted by the propelled virus vials once it drops into the ground is the death sound of the Big Snail.
- Like all Metal Slug 4 bosses, it was made using old sprites from previous Metal Slug games.
- If the player looks closely at its neck after the robot's face is destroyed, a reused sprite of the LV Armor can be seen, with a Rebel riding inside of it.
- It is unknown whether this Rebel is the one manning Big John or just some of the weapons of the boss.
- Though Aeshi Nero's lower part was never truly shown in-game for the most part, Big John seems to fulfill this role since this boss takes the form of the lower part of Aeshi Nero.
- The rebel LV Armor is only seen when Big John's real face appears.
- Due to a strange typo in Metal Slug Defense and Metal Slug Attack, Big John, along with the other Metal Slug 4 enemies and bosses, they are placed in the Independent faction.
- Big John's official spelling is revealed in Metal Slug Defense Ver. 1.26.
- Being a stationary unit, Big John is placed on the field through a miner.
- The zombie vials will not turn targets damaged by them into zombies, it will instead damage them.
- After taking enough damage, like its original counterpart, it shows its face after taking half a damage.
- If one looks closely, Big John's actual face can be seen glitched through its puppet form. This will happen in combat.