C4
The C4 is Snake's down special move. Snake places a C4 on the ground and may detonate it at any distance by pressing the button again (meaning that only one C4 can be used at once); however, after (roughly) 27 seconds, it detonates automatically, without help from Snake. When Snake detonates it, he flourishes the remote detonator, and exclaims "Now!" as he presses the button. The explosion can KO opponents at medium-high percentages.
When Snake says "Now!", he is attempting to detonate the C4, but this can be prevented by hitting him to interrupt the detonation. The C4 has about a 2-character explosion radius, and requires the target to be standing very close to it for the attack to be effective. It's rather easy to escape, and can also be guarded against.
The C4 can be placed directly on opponents, as long as they are within Snake's reach. It acts similar to a Gooey Bomb in that it can be passed from person to person when they get near each other, but after some time it may fall off. If standing next to one, Snake can place C4 on an adjacent wall in a similar fashion to that of the Metal Gear Solid series. C4 can also be placed on teammates for a "suicide bomber" effect.
While in mid-air, C4 can be dropped and will land on the ground directly below Snake, regardless of Snake's horizontal momentum. This makes for a more tactical, subtle placement of the explosive, and can be used to place C4 without drawing attention onto oneself. If Snake uses C4 directly on opponents at high damage percentages (at around 85%), it can lead into his up tilt then the C4 again, similar to SSB's Pikachu's thunderspiking except it doesn't KO at low percentage. C4 KOs almost as quickly as a fully charged landmine and can easily Star KO opponents when detonated on them in midair. Caution should be taken as to not make contact with the opponent as the C4 may stick to Snake when attached to the opponent. Using his up smash, grenades, or any moves that is distant from Snake's character model that propels the enemy upwards is usually sufficient enough. Snake's mortar is extremely helpful because he can perform a DACUS to quickly cover a lot of ground and surprise the opponent while knocking them in the air. The mortar usually has enough force and hitstun to prevent on opponent from air dodging the explosion. This guarantees KOs as low as 60%. Snake can recover his Cypher if he catches himself in the explosion of his own C4, giving him an almost infinite recovery (almost infinite because the C4 can KO him on high damages).
There is approximately a 1 in 6 chance that the C4 pack Snake uses will be a C3 explosive, which is butterfly-shaped, but the attack remains unchanged. The C3 butterfly is from Metal Gear Solid 3: Snake Eater. Naked Snake (Big Boss) models a C3 pack into a butterfly, throws it into in the air, catches it, and exclaims "Gotcha this time!" before slamming it against a liquid fuel container. This scene is a reference to an earlier event in which Snake is unable to grab a butterfly out of the air due to a lack of depth perception after losing his right eye.
Origin
Composition C-4 is one of the most common forms of plastic explosive (aka plastique) in the world. Like all plastic explosives, the C4 has the consistency of clay, allowing it to be molded into any shape and is incapable of being detonated by anything other than the detonator itself, making it safer. Unlike other plastic explosives, such as the C3, a C4 retains its clay-like consistency in all conditions. These properties have led to it being used worldwide, particularly in the US Army. The C4 is a recurring item in the Metal Gear series, and it is always required to progress through the game.
