GEAR FOURTH

Luffy coats his arm in Busoshoku Haki before biting into his forearm. Similar to how he activates Gear Third, he blows an incredible amount of air into his body, but this time he inflates his muscular structure before distributing the air throughout his body, with emphasis on his upper half.^[1]

Luffy inflates his muscles.

Luffy's proportions become warped and his body becomes much larger in size, with his arms, upper torso, and legs coated in Busoshoku Haki. The coating across his torso resemble the wispy ends made from flames, giving it the appearance of tattoos, and he gains shading around the inner and outer edges of his eyes. He also physically grows taller, standing around 4 meters tall. Similar to when using Gear Second, Luffy is continuously emitting steam in this form. As a side-effect, he is incapable of standing still on the ground, and instead constantly bounces on the spot. This form was created in order to combat the many powerful and large animals on Rusukaina.^[1] This form is based on the fierce guardian entity called the Nio Statue,^[3] which fits well with Luffy's will to protect his crewmates with his life and beyond. It also resembles his appearance when he became Nightmare Luffy (except for the hardened Busoshoku Haki 'tattoos', the enlarged biceps and a more rounded torso) due to Luffy's increased muscle size, haircut's shape and sinister expression.^[4]
On Volume 79's cover, Luffy's skin is shown to take a pinkish tone (similar to Gear Second in the anime). However, in the anime, though Luffy's uncoated skin is unchanged, the Busoshoku tattoos take on a reddish tint. Furthermore, he seems to take up some kabuki-like mannerisms, like exaggerated head-rolling, and occasionally dragging out words.^[5]
However, due to his characteristics upon entering this form, his enemies may see it as a joke and start to mock him, as seen when Doflamingo laughed at this form upon seeing it.

Luffy's arm changes direction in midair.

While Luffy is using Gear Fourth, his physical strength and speed are increased to the point where he is able to overwhelm a member of the Shichibukai, Donquixote Doflamingo, whereas before, Gear Second's lack of strength^[6] and Gear Third's lack of speed^[7] were ineffective in the fight. Unlike the previous forms, Gear Fourth focuses on compression more than stretching. While the compression grants Luffy's moves overwhelming speed and power, the range of his attacks is severely decreased. For example, the Gomu Gomu No Bazooka is a technique that Luffy has proven he can execute over varying distances, but due to needing to actually compress his fists for the Leo Bazooka, it required him to be right in front of Doflamingo. When the latter unveils his Devil Fruit Awakening, Luffy expresses his annoyance at his difficulty getting close to the Shichibukai. However, he can still deliver long-ranged attacks by stretching the part of the body he wants to attack with while retaining great power and speed, as his body is still greater in size and covered in Busoshoku Haki. Luffy also shows a much more versatile side of the Gomu Gomu no Mi's powers by changing the direction of his attacks mid-strike, by bouncing off the air, to hit a moving opponent without losing momentum.^[1]

Doflamingo's Busoshoku-enhanced kick bounces off of Luffy.

According to Doflamingo, the power of Luffy's attacks is greatly magnified in this form. His durability is also enhanced: despite being hardened with Busoshoku Haki, his body is still rubbery, so physical blows (even ones enhanced with Busoshoku Haki) simply bounce off of him.^[1]
This form, much like Gear Second, seems to utilize the principles of Rokushiki. It possesses both high speed movement reminiscent of Soru and the ability to kick off the air, similar to Geppo, cleverly adapted to work even better with his body's elasticity: Luffy not only bounces off the air, but allows attacks to bounce off his own body. His body is also now strong enough to defend against cutting attacks head-on.^[1]
However, there appears to be a limit to how long Luffy can keep Gear Fourth active, as noted by both Luffy himself and Doflamingo.^[8] Once this limit is reached, Gear Fourth is automatically deactivated, leaving Luffy exhausted and barely able to move. He is also left unable to use Haki for ten minutes after using this technique as it quickly depletes his Haki reserves.^[9] Even afterwards, Gear Fourth is extremely taxing on Luffy's body and stamina; after regaining his Haki from using Gear Fourth once, he barely had enough strength left to fight back.  After his brief use of it the second time, it rendered him unconscious for 3 days. After this, he seemed to mimick Ace's habits, namely his narcolepsy. Also, after he lands on the ground after using an attack, he cannot seem to control his landing due to warped proportions his body takes and bounces like a ball in a comical fashion until he either loses momentum or he somehow regains control and starts "flying" again.

SUMBER:
http://onepiece.wikia.com/wiki/Gomu_Gomu_no_Mi/Gear_Fourth_Techniques