Four Arms is a humanoid alien that is approximately 12 feet tall, has well developed muscles, two pairs of four-fingered arms and red skin. The black stubble stripe goes from the bridge of his nose to his neck. A second black stripe goes from his chin to his lower lip, and he has four eyes: a main pair and a smaller pair below them.
In the original series, Four Arms wears a white t-shirt with a black line going down it, black pants and fingerless gloves. He wears the Omnitrix symbol on his upper left shoulder.
In Ultimate Alien, Four Arms wears gold wrist and ankle bands, and wears a black underwear that have a gold waistband and two golden sashes going across his chest. His eyes are now green, he has a black ponytail and he wears the Ultimatrix symbol on his chest.
Gwen as Four Arms has her pants, her chest is light blue, her arms, neck and back are dark blue. Her skin is darker red and she has Gwen's hair. She bears the Omnitrix symbol on her upper left shoulder.
In Ben 10-Generator Rex: Heroes United, Four Arms, in addition to his clothes in Ultimate Alien, he now also has his pants, fingerless gloves, and tattoo like stripes on his body, which are the same as the stripes of his shirt in Original Series, giving him the look of Four Arms in the original series.
In Omniverse, Four Arms now has 3 short spikes on the top of his head, a mustache, the black stripe on the top of his head is gone, he has no hair, he has a black tank top like shirt as a muscle man at a circus would have with one large green stripe in the middle, and black pants. He has a belt where the Omnitrix symbol is located. He also has fingerless gloves like in the Original Series with green cuffs at the wrist of his gloves. His skin is also lighter than it was in Ultimate Alien.
11-year-old Ben's Four Arms in Omniverse looks the same as the original series but now has green eyes and the Omnitrix symbol on his shoulder is now green. In both his 11-year-old and 16-year-old incarnations, Four Arms now appears to have a second set of pectorals for his other set of arms.
Four Arms' most prominent ability is his super strength, being able to lift several times his own weight, punch through various resilient materials and even create strong shockwaves by clapping his four hands together or slaming his fists on the ground. He can jump several times his own height.
His strength allows him to use anything heavy as a weapon, from big rocks he can throw several feet away to beams and pillars that can be used as clubs. Another feature is his endurance and resilient skin, being almost fire and laser-proof.
Sometimes Four Arms' bulk is a disadvantage; all the extra muscle mass makes him relatively slow, and his size, likewise, makes using things designed for smaller species difficult.
When under the effects of a cold, Four Arms' strength and stamina are weakened, he develops pungent hives in his armpits, and his skin color turns a darker shade of red. [Source: Wikia]