Fourth Armor

The X4/Fourth Armor (フォースアーマー), sometimes called the Force Armor, is an all-around armor found first in Mega Man X4. In its debut game, it was collected in four parts and slowly became more powerful as the parts were collected, as in the first three games.

Armor Parts
Helmet Part, in Cyber Peacock's stage: If you complete the third section in the first part of the level fast enough to receive an S rating, the Helmet Capsule will be your reward.
 * Increases X's special weapons capacity, allowing him to fire endless uncharged special weapons and use less energy when firing charged weapons.

Body Part, in Magma Dragoon's stage: Found in a passage above and to the right of where the Ride Armor is found. It is reached by dash-jumping in the ride armor from the highest point and ejecting from it in mid-air. Enter the passage and break the wall with a charged Twin Slasher to find the Body Capsule.
 * Allows X to use a Giga Attack, the Nova Strike. It is a rushing, invincible, damage-inducing attack, with limited energy that is restored over time. X's damage taken is also cut in half.

Arm Part, in Storm Owl's stage: In the second part of the stage, right before the laser-based miniboss, you may notice a spike-lined vertical passage. To reach the top, fire the Lightning Web right in front of one of the spiky walls and use it to ascend the passage. At the top, you may choose one of two capsules:
 * Capsule 1: Allows X to charge up four charged shots at once. Four icons of plasma will appear next to X's life gauge that are used to keep track of the number of X's shots. Whenever X fires while an icon is visible, he will fire a charged shot.
 * Capsule 2: Upgrades X's charged shots to the Plasma Shot. This shot is much larger and more powerful than the standard blast, and will leave damaging orbs in its wake whenever it comes into contact with an enemy.

Leg Part, in Web Spider's stage: Keep to the right throughout the beginning of the stage. You'll find a break in the wall, and the Leg Capsule will be within it.
 * X can air-dash, and he can also hover by double-jumping. He can move while hovering, but not much.

The Force Armor can also be obtained in Mega Man X5. If X is chosen at the Character Select screen at the beginning, he will begin the game with the Fourth Armor. The second Arm Capsule is chosen, enabling the Plasma Shot, but the effects of the Helmet Part are not in effect for the duration of the game. The Nova Strike is also unavailable. If Zero is chosen instead, the Fourth Armor will become damaged in the first stage and be unusable for the remainder of the game.

Trivia

 * The Fourth Armor's second name, the "Force Armor", is a mistake brought about by the X5 bandai model kit of the same name. X5 itself clearly calls it "Fourth", as it is a replica of the fourth armor X accuired.
 * In Mega Man X4, the colors of the Arm Parts for the Fourth Armor are different when equipped with either the Stock Shot or the Plasma Shot in order to distinguish the two. For the Stock Shot, the forearms are white and the hands are blue. For the Plasma Shot, however, the forearms are black and the hands are red.
 * In Mega Man X5, however, the completed Fourth Armor's Arm Parts are white-and-blue and the type of shot that is built-in is the Plasma Shot.
 * Since the Ultimate Armor is the most recurring armor in the series (and the fact that it is essentially an upgrade to the Fourth Armor), most of the Fourth Armor's abilities (Plasma Shot, Nova Strike and Hover) were also the most recurring abilities in the series.